THE  UNIVERSITY 
OF  ILLINOIS 
LIBRARY 


5  8  U  94  8 


DEC  7  1960 

".*■  * 


J: 


GENERAL  AND  PHYSIOLOGICAL  FEATURES  OF  THE 
VEGETATION  OF  THE  MORE  ARID  PORTIONS 
OF  SOUTHERN  AFRICA,  WITH  NOTES  ON 
THE  CLIMATIC  ENVIRONMENT 


BY 

WILLIAM  AUSTIN  CANNON 


V 


the  libbari  of  m 

OCT  6  1924 

UNIVERSITY  OF  ILLINOIS 


Published  by  the  Carnegie  Institution  op  Washington 

Washington,  August,  1924 


CARNEGIE  INSTITUTION  OF  WASHINGTON 

Publication  No.  354 


Copies  of  this  book 
first  issued 
SEP  5  1924 


PRESS  OF  GIBSON  BROS.,  INC. 
WASHINGTON,  D.  C, 


/  £  l^e,  c.  •  £  ^  /G 


CONTENTS 


5H.US 

ULd 


W 


PAGE. 


Introduction .  3 

List  of  species  and  genera .  10 

General  features  of  the  climate  of  southern 

Africa .  11 

Temperature .  14 

Rainfall  in  southern  Africa .  16 

General  conditions .  16 

Rainfall  in  the  Little  Karroo .  24 

Rainfall  in  the  Great,  or  Central, 

Karroo .  25 

Aberdeen  and  Graaf  Reinet .  25 

Beaufort  West .  27 

Laingsburg .  27 

Matjesfontein .  28 

Drought  periods .  30 

Rainfall  in  the  Protectorate  of  Southwest 
Africa  and  in  the  northwestern  part  of 

the  Union .  32 

O’okiep .  32 

Warmbad .  33 

Keetmanshoop .  33 

Bethany .  34 

Gibeon .  34 

Windhoek .  34 

Gobabis .  34 

Karibib .  34 

Swakopmund .  35 

Luederitz  Bay .  35 

Drought  periods  in  the  Protectorate ....  35 

Effective  precipitation .  36 

Moisture  of  the  air .  38 

Seasonal  variation  in  relative  humidity .  38 

Winds .  40 

Evaporation .  42 

Ratio  of  rainfall  to  evaporation  (P/E) ...  44 

Atmometry  in  southern  Africa .  44 

National  Botanic  Gardens . '.  45 

Grahamstown .  47 

Pietermaritzburg .  48 

Matjesfontein .  48 

Beaufort  W’est .  49 

Swakopmund .  51 

Pretoria  and  Irene .  52 

Summary .  54 

General  features  of  the  vegetation,  espe¬ 
cially  of  the  more  arid  portions  of 

southern  Africa .  55 

Botanical  regions  of  southern  Africa .  58 

On  characteristics  of  the  vegetation  in 
portions  of  the  Namib  Desert  and  of 

the  Central  Karroo .  62' 

The  Namib .  62- 

The  Central  Karroo .  65 

Beaufort  West .  65 

Prince  Albert  Road .  67 

Matjesfontein .  68 

Notes  on  root  habits .  72 

Summary .  75 

Some  features  of  foliar  structure .  77 

Notes  on  leaf  structure .  78 

Aloe  variegata .  78 

Asparagus  striatus .  80 

Protea  neriifolia .  81 

Antizoma  capensis .  82 

Galenia  africana .  83 

Cadaba  juncea .  83 

Grewia  cana .  85 

Rhus  viminalis .  85 


pruc.-  'mm r 

v3  *.3 


Notes  on  leaf  structure — Continued.  page. 

Rhus  sp .  86 

Gymnosporia  buxifolia .  87 

Cussonia  spicata .  87 

Bauhinia  marlothii .  90 

Sutherlandia  frutescens .  90 

Cotyledon  paniculata .  91 

Stachys  sp .  92 

Aptosimum  indivisum .  92 

Carissa  ferox .  93 

Asclepias  filiformis  (?) .  94 

Euclea  undulata .  95 

Royena  pallens .  96 

Eriocephalus  sp .  96 

Euryops  lateriflorus .  98 

Pentzia  virgata .  99 

Pteronia  flexicaulis .  99 

Pteronia  incana .  100 

Relhania  squarrosa .  100 

Stoebe  sp .  101 

General  summary  and  discussion  of 

leaf-structures .  102 

Epidermis .  103 

Stomata .  106 

Trichomes .  107 

Mesophyll .  108 

Conductive  tissues .  109 

Notes  on  the  origin  of  foliar  structures.  110 

Proteacese .  110 

Menispermaceae .  Ill 

Aizoaceae .  Ill 

Capparidaceae .  112 

Tiliaceae . .  113 

Anacardiaceae .  113 

Celastraceae .  114 

Araliaceae .  114 

Leguminosae .  115 

Papillionatse .  115 

Caesalpinae .  116 

Crassulaceae . . .  116 

Labiatae .  117 

Scrophulariaceae .  117 

Asclepiadaceae .  118 

Apocynaceae . -  118 

Ebenaceae . .  119 

Compositae .  119 

Some  conclusions .  120 

Observations  on  the  foliar  transpiring 

power  in  winter  and  spring .  122 

Beaufort  West .  123 

Aloe  schlechteri .  123 

Gasteria  disticha .  124 

Grewia  cana .  125 

Gymnosporia  buxifolia  (?) .  127 

Massonia  latifolia .  128 

Matjesfontein .  129 

Aloe  striata .  129 

Cotyledon  coruscans .  130 

Cotyledon  paniculata .  131 

Eucalyptus  globulus  (?) .  135 

Euryops  lateriflorus .  137 

Euclea  undulata .  138 

Rhus  sp.  and  R.  viminalis .  139 

Protea  neriifolia .  140 

Namib .  141 

Welwitschia  mirabiiis .  141 

Bauhinia  marlothii .  142 

Summary .  142 

Generalized  summary .  145 


hi 


ILLUSTRATIONS. 

PLATES. 


1,  a.  Welwitschia  mirabilis  showing  character  of  habitat,  ca.  40  km.  east  of  Swakopmund, 

looking  toward  the  Swakop  River. 

b.  Acanthosicyos  horrida  in  bottoms  of  the  Swakop  River,  15  km.  east  of  Swakopmund. 

2,  a.  Welwitschia  mirabilis ,  Namib  Desert,  ca.  40  km.  east  of  Swakopmund.  Female 

plant. 

b.  Male  plant  of  Welwitschia  mirabilis ,  habitat  of  A. 

c.  Leaves,  natural  size,  of  Zygophyllum  stapfii. 

3,  a.  Vegetation  in  the  Welwitschia  habitat,  ca.  50  km.  east  of  Swakopmund,  Namib 

Desert.  Asclepias  filiformis  (left),  Zygophyllum  stapfii,  and  Bauhinia  marlothii. 
Looking  south. 

b.  Vegetation  in  the  Welwitschia  habitat.  Looking  north,  toward  the  Swakop  River. 

The  dark  shrublets  are  Zygophyllum  stapfii.  Asclepias  filiformis  (?)  is  in  the 
bottom  of  the  shallow  wash. 

c.  Zygophyllum  stapfii  in  the  habitat  of  Welwitschia  mirabilis,  about  40  km.  east  of 

Swakopmund,  Protectorate  of  Southwest  Africa. 

4,  a.  Arthrcerua  leubnitzii,  about  18  km.  east  of  Swakopmund,  Namib  Desert.  Plain 

south  of  the  Swakop  River. 
b.  Branch,  natural  size,  of  Arthrcerua  leubnitzii. 

5,  a.  Tree  type  in  Low  veld  near  Messina,  northern  Transvaal,  rainfall  about  25  inches, 

of  which  about  90  per  cent  is  in  summer.  Sesamnothamnus  lugardii  (?). 

b.  Adansonia  digitata,  near  Messina,  northern  Transvaal,  July,  showing  enormous 

development  of  stem  which  constitutes  a  water-storage  organ  of  great  capacity. 
Rainfall  about  25  inches,  of  which  about  90  per  cent  is  in  summer. 

c.  Euphorbia  cooperi  by  the  Zoutpansbergen,  rainfall  35  inches  or  over,  of  which  about 

90  per  cent  occurs  in  summer. 

6,  a.  View  of  veld,  looking  southwest  from  kopje  near  Beaufort  West,  Central  Karroo. 

b.  South  face  of  doloritic  kopje,  near  Beaufort  West,  Central  Karroo. 

c.  Bulboid  squat  stem  of  Adenia  schlechteri,  resting  freely  on  surface  of  ground,  with 

water-storage  capacity.  Low  veld  near  Messina,  northern  Transvaal.  Rain¬ 
fall  about  25  inches,  90  per  cent  in  summer. 

7,  a.  Gymnosporia  buxifolia  (?)  on  kopje  near  Beaufort  West.  Used  in  transpiration 

studies. 

b.  Branch  with  leaves,  one-half  natural  size,  Grewia  cana,  from  kopje  near  Beaufort 

West.  Used  in  transpiration  studies. 

c.  Leaves  and  spines  of  Gymnosporia  buxifolia  (?),  one-half  natural  size.  From  kopje 

(see  plate  6b)  near  Beaufort  West,  Central  Karroo. 

8,  a.  Aloe  schlechteri  on  north  slope  of  kopje  near  Beaufort  West. 

b.  Euphorbia  stellcespina  on  north  slope  of  kopje,  near  Beaufort  West. 

c.  Quadrat  No.  1,  on  south  slope  of  kopje  (compare  plate  6b),  near  Beaufort  West. 

The  following  are  included  among  the  perennials  occurring  in  the  area :  Carissa 
ferox  (?),  Euphorbia  mauritanica,  Grewia  cana,  Lycium  sp.,  and  Mesembryan- 
themum  sp. 

9,  a.  Gasteria  disticha  growing  at  base  of  Euphorbia  mauritanica  (?)  on  upper  south  slope 

of  kopje  near  Beaufort  West.  Used  in  transpiration  studies. 
b.  Massonia  latifolia  at  base  of  Lycium  sp.  on  slope  of  kopje  near  Beaufort  West. 
Used  in  transpiration  studies. 

10,  a.  Crassula  quadrangularis  growing  at  base  of  Lycium  sp.  on  upper  face  of  kopje  near 

Beaufort  West. 

b.  Senecio  longifolius,  by  water  hole,  6  miles  east  of  Beaufort  West. 

11,  a.  Cotyledon  decussata  growing  at  the  base  of  Lycium  sp.,  Prince  Albert  Road,  Central 

Karroo. 

b.  Mesembryanthemum  calamiforme-Cotyledon  hemisphaerica  (?)  community,  Prince 
Albert  Road,  Central  Karroo.  Rainfall  4.57  inches;  60  per  cent  in  summer. 

12,  a.  Quadrat  No.  3,  near  Matjesfontein,  looking  toward  the  Wittebergen.  There  were 

330  individuals,  perennials,  on  the  area,  10  by  10  meters,  about  equally 
divided  between  succulents  and  sclerophylls. 


IV 


ILLUSTRATIONS. 


V 


12,  b.  Veld  near  Matjesfontein,  looking  toward  Ngaap  kopje.  Mesembryanthemum 

spinosum,  M.  spp.,  Pentzia  virgata  dominate. 

13,  a.  Euphorbia  mauritanica,  veld  near  Matjesfontein. 

b.  Euphorbia  eustacei,  among  rocks,  near  Matjesfontein. 

14,  a.  Acacia  karroo,  in  winter  (August),  near  Matjesfontein. 

b.  Detail  of  A  showing  marked  spiniferous  character  of  branches,  which  is  especially 
noticeable  during  the  leafless  condition. 

15,  a.  Meseyibryanthemum  junceum,  on  veld  near  Matjesfontein. 

b.  Euryops  lateriflorus  by  rocky  outcrop  near  streamway,  Matjesfontein.  Used  in 
transpiration  studies. 

16,  a.  Cotyledon  wallichii  on  low  kopje  near  Matjesfontein. 

b.  The  leaf  succulent  Cotyledon  coruscens  on  slope  of  low  kopje  near  Matjesfontein. 
In  flower.  One  specimen  of  C.  paniculata ,  stem  succulent,  is  shown  in  middle 
ground  at  left. 

17,  a.  Crassula  perfossa  with  Cotyledon  orbiculata  at  back.  Grewia  cana  at  left.  Kopje 

near  Matjesfontein. 

b.  Vegetation  on  north  slope  of  kopje,  near  Matjesfontein.  Euphorbia  mauritanica 
in  middle  foreground,  with  Cotyledon  orbiculata  at  left  in  middleground.  Aloe 
striata  (?)  in  middle  ground  and  background.  Mesembryanthemum  spinosum  and 
Crassula  perfossa  dominating.  Grewia  cana  and  Lycium  sp.  at  left  and  right  in 
background. 

18,  a.  Cotyledon  wallichii  on  veld  near  Matjesfontein. 

b.  Cotyledon  paniculata,  stem  succulent,  on  kopje  near  Matjesfontein,  Mesembry¬ 
anthemum  junceum  (?)  dominating. 

19,  a.  Protea  neriifolia  at  Tweedside,  west  of  Matjesfontein,  used  in  transpiration  studies. 
b.  Vegetation  of  kopje,  near  Matjesfontein.  Mesembryanthemum  junceum  in  flower  in 

middle  ground;  Aloe  striata  (?)  with  dead  flowering  stalks  on  either  side.  Small 
specimens  of  Cotyledon  paniculata  at  right  in  foreground  and  at  left  in  middle 
ground.  Euphorbia  mauritanica  in  right  middle  ground  with  Euclea  undulata 
behind. 

20,  a.  Streamway  vegetation,  near  Matjesfontein.  Rhus  viminalis,  in  middle  ground, 

with  Acacia  karroo  in  front,  as  a  shrub,  and  on  the  left  as  trees. 
b.  Lebeckia  psiloloba  on  edge  of  village,  Matjesfontein.  It  occurs  in  some  numbers 
on  kopjes  a  few  miles  west. 

21,  a.  General  view  of  quadrat  No.  4,  near  Matjesfontein.  Mesembryanthemum  spinosum 

and  Pentzia  virgata  dominant.  Asparagus  capensis.  Out  of  397  individuals, 
perennials,  184  are  succulents. 

b.  Vegetation  on  lower  slope  of  foothills  near  Whitehill,  3  miles  east  of  Matjesfontein, 
Central  Karroo.  Euphorbia  mauritanica ,  left  foreground;  Crassula  portulacea, 
middle  ground;  Asparagus  sp.,  Rhus  sp.,  and  Euclea  undulata. 

22,  a.  Root  exposure  of  Galenia  africana  (left)  and  Eriocephalos,  showing  characteristic 

deep  penetration  in  both  species,  with  prominent  development  of  superficial 
roots  in  the  latter.  Matjesfontein,  near  streamway. 
b.  Euphorbia  stolonifera  on  rocky  portion  of  veld,  near  Matjesfontein. 

23,  a.  Prominent  development  of  superficial  roots  in  Asparagus  sp.  on  veld  near  Matjes¬ 

fontein.  Mesembryanthemum  spinosum  at  left  and  immediately  back  of  the 
small  Asparagus  shoot. 

b.  Root  exposure  of  Euryops  lateriflorus  by  small  wash,  7  miles  west  of  Matjesfontein. 
The  small  shrubs  in  the  background  are  in  part  Galenia  africana. 

24,  a.  Superficial  and  fairly  meager  root  system  of  Euphorbia  stolonifera  exposed  in  part 

by  erosion,  with  Cotyledon  (?),  in  shadow,  and  Mesembryanthemum  spinosum  in 
background. 

b.  Exposure  of  roots  of  Cotyledon  canescans  by  small  wash  near  Matjesfontein.  There 

were  two  main  roots,  both  superficial,  with  numerous  short  roots.  One  of  the 
main  roots  lies  on  the  surface  of  the  ground  in  the  foreground,  and  the  other  is 
to  be  seen  in  front  of  a  sheet  of  paper  back  of  the  plant. 

c.  Root  system,  removed  from  soil,  of  Cotyledon  coruscans,  showing  its  meager  develop¬ 

ment.  The  roots  are  mainly  superficial.  Veld  near  Matjesfontein. 

25,  a.  Euphorbia  multiceps  showing  prominently  developed  tap  root,  veld  at  Matjesfontein. 

b.  Euphorbia  multiceps ,  veld  at  Matjesfontein. 

c.  Aloe  variegata  in  flower,  veld,  near  Matjesfontein. 


VI 


ILLUSTRATIONS. 


26,  a.  Root  system  of  Mesembryanthemum  junceum  showing  the  prominently  developed 

superficial  roots.  Ca.  one-third  natural  size.  Veld,  near  Matjesfontein. 

b.  Mesembryanthemum  spinosum  showing  characteristically  marked  development  of 
superficial  roots.  Flats  south  of  Whitehill,  3  miles  east  of  Matjesfontein. 

27,  a.  Elytropappus  rhinocerotis,  near  streamway,  Matjesfontein,  showing  prominently 

developed  superficial  roots,  of  the  generalized  root  system,  exposed  by  erosion. 
b.  Root  exposure  in  Lycium  sp.  growing  by  stream  near  Matjesfontein,  showing  vege¬ 
tative  reproduction  from  superficial  lateral,  and  strongly  developed  tap-root. 

28,  a.  Anacampseros  papyracea,  one-half  natural  size,  Whitehill,  3  miles  east  of  Matjes¬ 

fontein. 

b.  Androcymbium  sp.,  one-half  natural  size,  veld,  Matjesfontein. 

c.  Crassula  columnaris  in  right  middle  ground,  in  flower,  and  in  preflowering  stage. 

Mesembryanthemum  pgymceum  (?)  at  left.  Ca.  one-fourth  natural  size,  veld, 
Matjesfontein. 

d.  Stapelia  pillansii  on  low  outcrop  near  Matjesfontein. 

29,  a.  Haworthia  sp.  showing  the  fleshy  and  short  superficial  roots,  veld,  Matjesfontein, 

natural  size. 

b.  Mesembryanthemum  pygmceum,  left;  young  Crassula  columnaris,  below;  Cotyledon  (?), 
right.  Two-fifths  natural  size. 

30,  a.  Young  plants  of  Cotyledon  paniculata,  natural  size,  showing  early  development  of 

succulency  in  the  stem,  veld  near  Matjesfontein. 
b.  Crassula  lycopodioides,  veld  near  Matjesfontein.  Natural  size. 

31,  a.  Young  plant,  one-half  natural  size,  of  Cotyledon  wallichii,  showing  the  early  develop¬ 

ment  of  succulency  in  the  stem  and  the  superficial  nature  of  the  meager  root 
system,  veld  near  Matjesfontein. 

b.  Pelargonium  crithmifolium  showing  prominent  development  of  tap  root,  one-half 
natural  size,  veld  near  Matjesfontein. 


TEXT-FIGURES. 

PAGE. 


1.  Midiwnter  and  midsummer  mean  isotherms.  The  shaded  contours  delimit 

approximately  the  4,000-foot  level .  14 

2.  Average  annual  rainfall.  In  part  from  Mem.  4,  Bot.  Sur.  So.  Africa,  1922 .  17 

3.  Seasonal  distribution,  in  percentages,  of  rainfall.  Adapted  from  Mem.  4,  Bot. 

Sur.  So.  Africa,  1922 .  19 

4.  Rainfall  for  January  1920.  Adapted  from  weather  report .  20 

5.  Rainfall  for  August  1920.  Adapted  from  weather  report .  22 

6.  Minimal  rainfall,  1885-1894.  In  part  after  Marloth,  Das  Kapland.  The 

heavy  line  running  northwest  from  near  Port  Elizabeth  approximately 
separates  the  region  of  summer  rains,  to  the  east,  from  that  of  winter 
rains .  22 


7.  Main  botanical  regions,  after  Pole  Evans.  I.  Karroo  province  with  position  and 

extent  of  Central  or  Great  Karroo  indicated  in  southern  portion,  with 
Upper  Karroo  north  of  the  escarpment.  II.  Namaqualand  desert 
province.  III.  Cape  region.  IV.  Kalahari  park  and  bush  province. 

V.  High  veld,  in  western  portion,  Steppe  and  forest  province  in  moun¬ 
tains  and  Eastern  grass  veld  and  Coast  forest  area  to  the  east .  59 

8.  a.  Cross-section  of  leaf  of  Aloe  variegata  to  show  the  heavy  outer  epidermal  wall, 

with  the  cuticularized  portion  indicated  by  dotted  lines,  and  the  deeply 
placed  stomata.  X300. 

6.  Asparagus  striata,  cross-section  of  leaf,  showing  the  heavy  epidermis  with 
thickened  outer  wall,  the  limited  development  of  palisades,  and  a  portion 
of  a  centrally  situated  mass  of  sclerenchymatous  tissue.  X300. 
c.  Protea  neriifolia ,  section  of  an  old  leaf,  in  which  is  indicated  the  heavy  epi¬ 
dermis  with  much  thickened  outer  wall  and  deeply  placed  stomata. 

The  pronounced  palisade  formation  is  indicated.  X300. 


ILLUSTRATIONS. 


VII 


PAGE. 

8.  d.  Section  of  leaf  of  Antizoma  capensis  showing  the  superficially  placed  stomata 

and  palisade  chlorenchyma.  X300. 

e.  Galenia  africana,  to  show  the  vesicular  epidermal  cells,  superficial  placing 
of  the  stomata,  and  character  of  the  outer  chlorenchyma.  X300. 

/.  Cross-section  of  branch  of  Cadaba  juncea  showing  the  deeply  placed  stomata 
with  marked  development  of  outer  vestibule.  The  heavy  character  of 
the  epidermis  is  indicated.  X300. 

g.  Cadaba  juncea ,  shoot,  fragment  of  section  taken  immediately  within  the 

chlorenchyma  to  show  the  tracheids.  X300. 

h.  Cross-section  of  leaf  of  Grewia  cana  showing  the  dorsi-ventral  symmetry  of 

structure.  The  heavy  epidermis  of  the  dorsal  side,  without  cover  tri- 
chomes,  and  the  light  epidermis  of  the  ventral  side  with  trichomes  are 
shown. 

i.  Grewia  cana,  fragment  of  cross-section  of  ventral  portion  of  leaf  showing 

the  superficially  placed  stomata.  X325. 

j.  Fragment  of  cross-section  of  leaf  of  Rhus  viminalis,  to  show  excessive  develop¬ 

ment  of  palisades.  The  thickness  of  the  leaf  is  indicated.  X 150. 

k.  Detail  of  epidermis  from  ventral  side  of  leaf  of  Rhus  viminalis  showing  char¬ 

acter  of  stomata  and  suggesting  that  of  the  spongy  chlorenchyma  be¬ 
low  it.  X325. 

l.  Rhus  sp.  Whitehill.  Fragment  of  epidermis  from  dorsal  surface  of  leaf,  to  show 

the  heavy  covering  of  resin  and  its  relation  to  the  base  of  trichome. 

X300. 

m.  Gymnosporia  buxifolia,  detail  of  epidermis  of  leaf,  longitudinal  section,  show¬ 

ing  superficial  placing  of  stomata  and  the  fairly  heavy  outer  epidermal 
wall.  Cuboid  sub-epidermal  cells,  in  effect  a  hypoderm,  separate  the 
palisades  from  the  epidermis.  X325 .  79 

9.  a.  Cussonia  spicata,  detail  of  epidermis,  and  showing  stomata  and  cuboid 

chlorenchyma.  Ventral  surface.  X300. 

b.  Cussonia  spicata,  dorsal  surface  of  leaf,  showing  heavy  outer  epidermal  wall 

and  hypoderma,  several  cells  in  thickness,  with  palisades  within.  X300. 

c.  Fragment  of  leaf  of  Cotyledon  paniculata,  prepared  from  material  grown  at 

the  Coastal  Laboratory,  showing  the  cuboid  chlorenchyma  and  delicate 
epidermis.  X70. 

d.  Cross-section  of  leaf  of  Stachys  sp.  showing  the  confused  mass  of  trichomes 

on  both  surfaces  and  the  prominently  developed  palisades  on  the  dorsal 
side,  with  spongy  parenchyma  on  the  ventral  side  and  stomata  with 
guard-cells  which  project  slightly.  X300. 

e.  Aptosium  indivisum,  cross-section  of  leaf,  showing  heavify  developed  outer 

epiermal  wall,  short  palisades,  and  superficially  placed  stomata.  X300. 

f.  Carissa  ferox,  fragment  of  ventral  side  of  leaf  showing  heavy  outer  epidermal 

wall,  with  superficially  placed  stomata,  having  two  subsidiary  cells. 

The  palisade-like  character  of  the  outer  chlorenchyma  of  the  ventral  side 
is  indicated.  There  probably  are  better  developed  intercellular  spaces, 
however,  than  are  shown  in  the  sketch.  X325. 

g.  Asclepias  filiformis  (?),  semi-diagrammatic  cross-section  of  leaf,  showing 

channelling  of  leaf  and  disposition  of  main  tissues.  The  two  separate 
masses  of  chlorenchyma  are  indicated.  For  further  explanation,  see  text. 

X70. 

h.  Asclepias  filiformis  (?),  detail  from  dorsal  side  of  leaf,  showing  the  heavy  outer 

epidermal  wall  and  character  of  stomata.  X300. 

i.  Euclea  undulata,  fragment  of  cross-section  of  ventral  side  of  leaf,  showing 

the  superficial  placing  of  the  stomata  and  character  of  the  epidermis. 

X300. 

j.  Euclea  undulata,  portion  of  cross-section  of  leaf  to  show  the  heavy  epidermis, 

prominent  development  of  palisade  cells,  and  scherenchyma.  X300. 

k.  Eriocephalus  sp.,  cross-section  of  leaf,  with  the  most  prominent  tissues  out¬ 

lined:  e,  epidermis ;fv,  conductive  tissue;  p,  chlorenchyma.  X225 .  89 


VIII 


ILLUSTRATIONS. 


PAGE. 

10.  a.  Eriocephalus  sp.,  cross-section  of  leaf,  to  show  the  absence  of  lumen  in  the 
epidermal  cells  because  of  the  secondary  thickening  of  the  walls.  The 
bases  of  two  trichomes  are  shown  and  the  pronounced  development  of 
aplisades  indicated.  X300. 

b.  Euryops  lateriflorus,  section  of  leaf  in  which  the  heavy  epidermis,  deeply 

placed  stomata,  and  short  palisades  are  indicated.  X300. 

c.  Pentzia  virgata,  cross-section  of  leaf  to  show  the  superficially  placed  stomata, 

relatively  thin  epidermis  with  heavy  outer  wall,  secreting  trichome,  and 
two-ranked  palisade  tissue.  X300. 

d.  Pteronia  flexicaulis,  cross-section,  showing  the  distribution  of  the  following 

tissues:  e,  epidermis;  fv,  conductive  tissue  wdth  sclerenchyma  in  circles; 
p,  chlorenchyma.  X150. 

e.  Pteronia  flexicaulis ,  cross-section  showing  the  heavy  epidermis  with  greatly 

thickened  outer  wall  and  superficially  placed  stomata.  X300. 

f.  Pteronia  incana,  cross-section  of  leaf,  in  which  the  following  are  shown:  d , 

ducts;  fv,  conductive  tissue;  p,  chlorenchyma  with  the  epidermis  without 
the  dotted  line.  X50. 

g.  Pteronia  incana,  cross-section  of  leaf  showing  heavy  outer  epidermal  wall 

and  relatively  short  palisades.  X300. 

h.  Royena  pallens,  cross-section  of  leaf  showing  secretion  of  the  surface  of  the 

epidermis,  the  superficially  placed  stomata,  and  palisades.  X300. 

i.  Stoebe  sp.,  semi-diagrammatic  cross-section  of  leaf,  showing  the  extent  of  the 

mass  of  trichomes  on  the  ventral  side,  within  the  curving  broken  line, 
the  width  of  the  epidermis  and  the  conductive  tissue  having  scleren¬ 
chyma,  indicated  by  circles,  on  either  side.  X60. 

j.  Stoebe  sp.,  cross-section  of  leaf,  showing  modified  dorsi-ventral  symmetry  of 

structure.  The  light  epidermis,  stomata  with  projecting  guard-cells,  and 
trichomes  of  the  ventral  side  are  sharply  contrasted  with  the  heavy 


epidermis,  the  absence  of  stomata  and  of  trichomes  of  the  dorsal  side. 

X300 .  97 

11.  Average  indices  of  foliar  transpiring  power  at  2-hour  intervals,  8h  a.  m.  to 

10h  p.  m.  A,  Aloe  schlechteri ;  B,  Gasteria  disticha;  C,  Aloe  striata;  D, 

C  otyledon  canescens .  130 

12.  Average  indices  of  foliar  transpiring  power  at  2-hour  intervals,  8h  a.  m.  to 

10h  p.  m.  A,  Cotyledon  paniculata;  B,  Massonia  latifolia .  131 

13.  Average  indices  of  foliar  transpiring  power  at  2-hour  intervals,  8h  a.  m.  to 

10h  p.  m.  A,  Protea  neriifolia;  B,  Rhus  viminalis;  C,  Euryops  lateriflorus; 

D,  Grewia  cana;  E,  Gymnosporia  buxifolia .  140 


GENERAL  AND  PHYSIOLOGICAL  FEATURES  OF  THE 
VEGETATION  OF  THE  MORE  ARID  PORTIONS 
OF  SOUTHERN  AFRICA,  WITH  NOTES  ON 
THE  CLIMATIC  ENVIRONMENT 


By  WILLIAM  AUSTIN  CANNON 


GENERAL  AND  PHYSIOLOGICAL  FEATURES  OF  THE 
VEGETATION  OF  THE  MORE  ARID  PORTIONS 
OF  SOUTHERN  AFRICA,  WITH  NOTES  ON 
THE  CLIMATIC  ENVIRONMENT. 


INTRODUCTION. 

The  remarkable  vegetation  of  Southern  Africa  has  received  the 
attention  of  many  scientists  and  the  leading  characteristics  have 
long  been  known.  But  many  features  of  the  flora  remain  for  investi¬ 
gation.  Undescribed  species  are  being  found.  The  origin  of  the 
vegetation  as  a  whole  and  that  of  prominent  plant  types  is  being 
sought.  Habitats  are  being  defined  and  possible  relation  of  species 
to  physical  environmental  factors  subjected  to  observational  and  ex¬ 
perimental  investigation.  All  of  this  is  at  present  being  done  both 
by  individuals  working  intensively  on  local  problems  and  in  a  larger 
way  by  the  Union  of  South  Africa  through  the  agency  of  the  Botanical 
Survey,  an  organization  which  is  pushing  forward  investigations  on 
subjects  possibly  not  always  best  suited  to  separate  and  individual 
effort.  Such  division  of  labor  makes  it  possible  for  the  visiting 
botanist  to  make  his  contribution  without  duplicating  work  already 
at  hand  and  with  hope  of  contributing  to  the  general  end  desirable 
of  attainment.  So  far  as  the  point  of  view  of  the  writer  of  these  pages 
is  concerned,  it  was  not  so  much  to  contribute  to  the  knowledge  of  the 
plants  per  se  as  to  measure  the  conditions  of  plant  life  and  the  char¬ 
acteristics  of  the  plants  of  the  more  arid  portions  of  the  country  with 
the  yardstick  of  his  experiences  in  other  arid  lands  that  led  to  the  visit 
to  the  Union. 

A  botanist,  seeing  some  of  the  different  arid  or  desert  regions  of 
the  world,  is  struck  with  the  obvious  differences  in  the  habit  of  the 
vegetation,  as  well  as  by  the  differences  in  the  habitats,  although  he 
may  be  quite  well  aware  that  there  may  also  be  certain  features  of  both 
which  widely  separated  arid  regions  may  hold  in  common.  He  is  very 
likely  to  be  convinced  finally  that  the  association  of  habit  and  habitat 
is  casual  and  to  a  degree  local,  but  that  the  more  remote  analogies  are 
casual  and  depend,  to  a  large  degree  but  not  wholly,  on  the  reaction  of 
plants  to  an  environment  which  may  either  be  like  or  unlike  another 
environment  with  which  it  is  compared.  That  the  arid  region  vege¬ 
tation  of  distinct  world  areas  with  similar  or  nearly  similar  climatic 
characteristics  should  ever  develop  along  unlike  lines  is  a  matter  for 
investigation. 


3 


4 


FEATURES  OF  THE  VEGETATION  OF  THE 


Although,  as  suggested,  the  rainfall  of  widely  separated  arid  regions 
may  possibly  be  the  same,  it  does  not  in  the  least  follow  that  there  is 
to  the  same  extent  a  parallel  in  the  vegetation.  For  example,  in  the 
arid  desert  regions  of  central  Australia,  especially  in  northern  South 
Australia,  one  encounters  shrubs  and  trees  with  leaves  and  leaf-like 
phyllodia  of  a  leathery  texture,  which  are  usually  of  a  small  size  and 
not  deciduous.  There  are  practically  no  succulents.  In  regions  with 
a  small  rainfall  in  southern  Algeria  there  are  also  no  or  few  succulents, 
but  both  deciduous  and  evergreen  species  occur.  In  the  drier  portions 
of  North  America  are  to  be  found  both  deciduous  and  evergreen  shrubs 
or  trees  and  succulents  as  well.  And,  finally,  in  southern  Africa  there 
is  a  great  variety  of  stem  and  leaf  succulents,  and  of  both  evergreen 
and  deciduous  trees  and  shrubs  growing  under  arid  conditions.  Appar¬ 
ently  the  absence  of  one  type  of  plant  from  any  given  arid  region 
does  not  necessarily  signify  that  it  will  not  survive  there  if  brought  in, 
as  the  disastrous  results  following  the  introduction  of  cacti  into  Queens¬ 
land  would  indicate.  Also,  it  is  of  interest  to  note  that  the  species 
of  Acacia  in  Australia  are  evergreen,  while  in  southern  Africa,  in 
southern  Algeria,  and  in  North  America  the  acacias  are  wholly  de¬ 
ciduous.  As  between  central  Australia  and  southern  Algeria  the 
rainfall  which  is  periodic  occurs  mainly  in  the  cool  season,  while  in  the 
two  other  regions  referred  to  where  species  of  the  genus  are  to  be  found 
the  rainfall  may  be  either  in  winter  or  in  summer,  or  both  in  winter  and 
summer.  The  cause  of  leaf  fall,  or  of  leaf  retention,  as  the  case  may 
be,  is  clearly  not  always  directly  traceable  in  all  of  the  regions  to  the 
same  characteristic  of  the  precipitation. 

In  one  respect,  if  not  in  many,  there  is  agreement  in  the  direction 
of  the  development  of  perennial  woody  plants  in  these  different  arid 
regions.  The  species  are  relatively  small,  or  at  least  the  surface  is 
often  greatly  reduced.  Leaves  may  be  wanting  wholly,  or  they  may 
be  present  in  juvenile  stages  only,  or  at  any  rate  they  usually  are 
narrow  and  usually  short,  and  not  greatly  dissected.  Such  conditions 
appear  to  be  traceable  to  the  immediate  effect  of  the  desiccating  power 
of  the  atmosphere  as  well  as  to  the  effects  of  intense  insolation,  and  such 
environmental  factors  are  present  in  all  arid  regions.  But  the  species 
may  be  unlike  as  regards  reactions  to  such  environmental  conditions. 
Some,  for  example,  have  the  power  of  giving  off  watery  vapor  much 
more  highly  developed  than  other  species  and  appear  not  to  be 
able  to  decrease  the  rate  of  water-loss  when  the  supply  falls  below 
the  demand,  and  when  not  to  do  so  may  mean  wilting  and  ultimate 
death. 

In  all  arid  regions  also  there  is  great  variation  in  the  temperature 
as  between  day  and  night  and  from  season  to  season.  This  is  due  to 
the  small  amount  of  moisture  present  in  the  atmosphere,  which  permits 
rapid  loss  of  heat  at  night. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


Having  given  different  species  with  unlike  characteristics  of  shoot 
and  root  already  developed,  it  is  not  difficult  to  understand  how  they 
may  be  affected  in  an  unlike  manner  by  great  variations  of  the  temper¬ 
ature  of  the  air,  by  intense  insolation,  and  by  low  relative  humidity. 
Where  the  transpiration  surface  is  small,  other  things  being  equal, 
the  loss  of  water  will  not  be  large,  but  owing  to  the  necessity  of  the 
manufacture  of  foods  on  the  part  of  the  shoot,  the  chlorophyll-bearing 
organs  must  still  retain  a  certain  expanse  for  the  proper  carrying  on 
of  this  function.  Where,  however,  the  light  is  intense  this  can  be 
relatively  small.  Various  devices  may  be  found  to  occur  in  plants  of 
arid  regions,  aside  from  the  reduction  in  the  surface,  by  which  the  same 
ends  are  attained;  that  is,  immunity  from  injury  from  excessive  desic¬ 
cation  and  excessive  solar  radiation,  while  at  the  same  time  photo¬ 
synthetic  activities  can  proceed.  But  such  modifications  affect 
directly  the  relation  of  the  shoot  to  temperature  as  well.  In  the 
case  of  xerophytes  of  arid  regions,  where  the  transpiration  is  not  large, 
the  cooling  is  largely  by  radiation  only  and  the  shoots,  particularly 
those  with  water-balance,  may  become  excessively  warm. 

The  three  environmental  factors  above  mentioned  directly  affect 
the  shoot,  but  only  the  general  moisture  relations  and  the  great 
temperature  changes  of  the  air  directly  affect  the  roots,  and  then  mainly 
through  the  medium  of  the  soil  in  which  they  are  placed.  Roots  of 
different  character,  as  whether  fleshy  or  fibrous,  sparse  or  copious, 
deeply  placed  or  superficially  so,  are  affected  in  an  unlike  manner  by 
these  atmospheric  factors  in  a  way  quite  analogous  to  the  shoot. 
Unlike  the  shoot,  however,  there  apparently  is  no  “xerophytic”  type 
of  root-system,  unless  the  meager  and  usually  shallowly  placed  roots 
of  most  succulents,  and  at  the  other  extreme  the  deeply  penetrating 
roots  of  species  where  there  is  sufficient  soil  and  an  accessible  water- 
table,  or  at  least  deeply  penetrating  water,  can  be  said  to  be  such. 
And  aside  from  the  roots  of  succulents,  including  root  succulents,  those 
of  species  growing  in  analogous  habitats  in  widely  separated  arid 
regions  probably  closely  resemble  one  another,  and  possibly  do  not 
exhibit  differences  attributable  to  or  associated  with  the  region  of 
which  they  are  native,  in  a  way  or  to  a  degree  at  all  analogous  to  the 
behavior  of  the  shoot. 

Organs  or  structures  peculiar  to  plants  of  extremely  arid  regions, 
as  compared  to  species  of  semi-arid  regions,  for  example,  are  doubtful. 
The  presence  of  trichomes,  the  reduction  of  the  transpiring  surface, 
the  orientation  of  the  leaves,  double  epidermis,  heavy  outer  cell- walls  of 
the  epidermis  or  its  cuticularization,  mucilaginous  cells,  as  well  as 
the  in-rolling  of  leaves,  and  the  great  development  of  cell- wall  material, 
aie  not  confined  to  species  of  the  most  arid  regions,  but  in  them  may 
find  the  most  complete  expression.  This  general  type  of  development, 
however,  which  is  probably  to  be  regarded  as  an  expression  of  the 


6 


FEATURES  OF  THE  VEGETATION  OF  THE 


reaction  of  the  plant  to  some  features  of  the  moisture  and  light  environ¬ 
ment,  more  especially,  may  be  so  highly  specialized  as  almost  to  con¬ 
stitute  special  structural  form.  Of  these  might  be  mentioned  the 
very  great,  almost  exaggerated,  cell-wall  development  in  many  Aus¬ 
tralian  sclerophylls,  as  well  as  the  curious  development  from  secretions 
in  Hakea  of  a  supra-epidermis  with  continuous  air-cavity  between  it 
and  the  true  epidermis,  and  provided  with  pores.  The  true  stomata 
open  into  the  cavity  and  thus  are  only  indirectly  connected  with  the 
atmospheric  air.1 

Although  thus  there  may  be  no  break  in  the  continuity  of  structures 
as  between  species  inhabiting  extremely  arid  regions,  and  those  in 
regions  better  favored  with  rain,  with  the  effect  that  there  is  no  type 
which  is  peculiarly  eremologous,  there  is,  however,  a  special  class  of 
plants  which  are  indigenous  to  regions  of  moderate  rainfall,  but  which 
may  extend  to  a  certain  degree  both  into  regions  of  smaller  and  of  larger 
precipitation.  Such  are  succulents,  or  species  having  the  capacity 
of  storing  water,  occasionally  in  large  amount,  because  of  the  organ¬ 
ization  of  cells  containing  mucilaginous  material.  An  apparent  char¬ 
acteristic  of  all  succulents  is  the  meager  development  of  the  roots. 
Moreover,  it  appears  that  the  roots  of  succulents  are  usually  placed 
near  the  surface  of  the  ground  and  do  not  penetrate  deeply,  irre¬ 
spective  of  its  depth.  The  destructive  effect  of  a  sudden  excess  of 
water,  whereby  cells  may  be  ruptured,  is  not  unknown  in  species  of 
this  type.  Under  natural  conditions,  possibly  as  a  consequence  of 
these  factors,  succulents  do  not  inhabit  extremely  moist  soils,  but,  on 
the  other  hand,  may  be  situated  in  soils  which  are  subject  to  periodic 
drying  and  those  which  may  be  shallow.  The  suggestion  seems  perti¬ 
nent,  although  put  forward  tentatively,  that  the  type  of  root  develop¬ 
ment  of  succulents,  the  usual  situation  of  succulents  with  respect  to 
depth  of  soil  and  its  moisture  content,  including  the  depth  to  the 
water-table  or  otherwise  perennially  wet  soil,  and  their  presence  in 
regions  having  periodic  rainfall,  are  all  dependent  on  the  presence  of 
mucilages  in  the  cells  of  such  plants.  An  extensive  root  system  with 
possibility  of  correspondingly  large  capacity  for  the  absorption  of 
water,  or  the  placing  of  the  roots  so  that  such  large  absorption  is 
especially  forwarded,  or  a  large  and  continuous  supply  of  water,  might 
operate  to  disorganize  the  tissue  as  above  suggested.  It  is  of  interest 
to  note  in  this  connection  that  such  harmful  effects  are  averted  in 
certain  cacti  by  the  organization  of  leaves  having  great  transpiring 
power  at  the  time  of  the  most  intense  vegetative  activity  and  when 
the  capacity  for  water  absorption  is  particularly  large.2 

The  present  study  is  the  third  of  a  series,  in  the  writer’s  minor 
research,  of  occasional  papers  on  the  botanical  features  of  arid  regions. 

1  Plant  habits  and  habitats  in  the  arid  portions  of  South  Australia.  W.  A.  Cannon.  Carnegie 
Inst.  Wash.  Pub.  No.  308,  1921,  p.  131. 

*  Biological  relations  of  certain  cacti.  W.  A.  Cannon,  American  Naturalist,  vol.  40,  p.  27,  1906. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


7 


It  was  orginally  planned  by  the  writer  to  include  in  the  series  as 
many  studies  on  distinctive  deserts  as  might  be  practicable,  to  be 
concluded  by  a  physiological-ecological  work  of  a  comprehensive 
nature  on  deserts  in  general.  In  a  research  of  this  scope  and  nature 
the  results  of  laboratory  experimentation,  by  myself  and  others,  are 
carried  into  the  field  to  explain  so  far  as  possible  the  great  variety  of 
features  associated  with  living  plants  there  observed.  While  thus  it 
is  possible,  as  well  as  desirable,  for  different  persons  to  provide  the 
experimental  background,  it  is  clearly  advantageous,  if  not  indis¬ 
pensable,  that  one  and  the  same  person  make  the  studies  in  the  field. 
A  comparative  view,  necessary  to  work  of  this  scope,  is  thus  obtained, 
and  there  are  other  advantages  which  need  not  be  dwelt  on  in  this 
place. 

To  attempt  to  apply  to  foreign  and  un visited  arid  regions  conclu¬ 
sions  narrowly  won  is  at  once  dangerous  and  may  be  highly  misleading. 
Each  separate  arid  region  has  problems  of  its  own  which  require  specific 
study.  Even  the  common  fact  of  aridity  is  exceedingly  complex, 
possibly  with  unlike  causes  and  characteristics  as  well  as  with  dis¬ 
similar  physiological  and  ecological  relations. 

Referring  now  to  the  general  course  of  the  studies  on  the  plants, 
and  their  environment,  of  southern  Africa  as  developed  in  this  paper, 
the  following  main  subjects  are  considered:  (1)  the  perennial  flora  of 
the  more  arid  regions;  (2)  the  climatic  environment;  (3)  plant  habits 
and  plant  habitats;  (4)  comparative  structure  of  leaves;  and  (5)  the 
transpiring  power  of  leaves. 

The  fact  that  Europeans  have  visited  and  lived  in  southern  Africa 
since  the  last  part  of  the  fifteenth  century,  and  that  not  only  has 
much  attention  been  given  to  the  interesting  flora,  but  that  weather 
records  have  been  kept  continuously  more  than  100  years,  are  of  de¬ 
cided  assistance  to  persons  interested  in  the  African  plants  and  in  their 
physical  environment.  Thus,  the  course  of  the  temperature  and  that 
of  the  rainfall,  at  least  for  certain  stations,  are  well  known;  but  as  to 
other  climatic  features,  particularly  evaporation,  the  data  are  not  so 
complete.  It  was  in  part  to  contribute  to  the  general  knowledge  of 
evaporation  in  the  arid  portions  of  southern  Africa,  but  more  by  way 
of  better  defining  special  habitats,  that  the  writer  carried  atmometers 
and  put  them  into  operation  there.  Through  the  cooperation  of 
several  persons,  the  instruments  were  set  up  and  read  at  different 
stations,  including  the  ones  visited  by  the  writer,  and  certain  compara¬ 
tive  results  were  obtained.  The  work  thus  begun  was  taken  over  by 
the  Botanical  Survey  of  the  Union  and  is  now  going  forward  under  its 
auspices. 

In  the  transpiration  studies,  the  Stahl- Livingston  cobalt-chloride 
method  was  employed,  and  the  form  of  apparatus  actually  used  was 


8 


FEATURES  OF  THE  VEGETATION  OF  THE 


especially  planned  and  developed  by  Livingston  for  the  South  African 
work.  As  constructed,  the  apparatus  was  found  to  be  easily  carried 
about,  and  to  be  safely  taken  on  treks  in  Cape  carts,  for  example, 
where  more  fragile  and  larger  apparatus  might  easily  be  damaged. 
The  importance  of  obtaining  comparative  transpiration  values,  in  work 
of  this  kind,  is  unquestioned.  Thus,  the  transpiring  power  of  the 
leaves  of  several  representative  species  was  observed,  some  of  which 
are  infrequently  seen  by  botanists.  Of  these  Welwitschia  mirabilis 
of  the  Namib  Desert  was  one.  The  demonstration  of  a  relatively  high 
index  of  transpiring  power  of  its  leaves  was  a  matter  of  interest  and 
appears  to  denote  the  essentially  schlerophyllous  nature  of  the  rare 
species. 

The  habit-habitat  studies  were  conducted  on  several  lines.  The 
local  distribution  was  observed  as  accurately  as  possible,  and  in  this 
quadrats  were  employed  and  the  camera  freely  used.  The  latter  also 
provided  the  means  of  preserving  plant  habits,  whether  of  shoot  or  of 
root.  As  to  special  root  studies,  however,  it  should  be  stated  that  the 
writer  was  regrettably  obliged  to  restrict  his  observations  to  a  rela¬ 
tively  few  species.  At  the  same  time,  it  was  keenly  realized  that 
possibly  no  region  exists  which  would  better  repay  intensive  studies 
on  roots  in  the  field  than  the  arid  portions  of  southern  Africa. 

The  perennial  species  of  arid  southern  Africa  are  more  or  less  closely 
related  to  species  now  occurring  in  humid  districts  not  far  distant. 
The  matter  of  the  direction  of  descent  need  not  be  considered  here. 
In  certain  families  the  direction  is  apparently  toward  the  more  xero- 
phytic,  but  in  others  it  is  in  the  opposite  direction.  However  this  may 
be,  a  study  of  structure  indicates  what  tissues  may  have  been  directly 
affected  by  the  arid  habitat,  in  what  way,  and  to  what  extent.  Thus 
in  some  instances  it  may  be  possible  to  connect  the  course  of  morpho¬ 
logical  development  with  physiological  processes  as  being  causative, 
although  in  other  instances  such  relation  may  not  be  clear.  New 
structures  have  doubtfully  arisen,  as  mentioned  above,  but,  on  the 
other  hand,  family  characteristics  can  often  be  identified  and  have 
apparently  survived  when  not  inimical  to  the  survival  of  the  species. 

In  a  study  like  the  present  one,  of  rather  limited  scope  and  greatly 
restricted  as  to  time,  it  is  impossible  to  go  far  without  the  cooperation 
of  botanists,  other  scientists,  and  non-scientific  but  interested  persons. 
Such  cooperation,  at  once  spontaneous  and  most  generous,  was  not 
in  the  least  wanting  in  the  present  instance.  Although  it  is  impossible 
to  acknowledge  specifically  all  of  the  favors  and  aid  received,  special 
acknowledgment  should  be  made  to  the  following :  In  connection  with 
the  atmometric  work  mainly,  Professor  R.  H.  Compton,  University 
of  Cape  Town  and  the  National  Botanic  Gardens,  Kirstenbosch ; 
Dr.  Halm,  Royal  Observatory,  Cape  Town;  Professor  S.  Schonland, 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


9 


Albany  Museum,  Grahamstown ;  Professor  J.  W.  Bews  and  Mr.  R. 
D.  Aitken,  Natal  University  College,  Pietermaritzburg;  Mr.  J.  Pat¬ 
terson,  Matjesfontein;  Mr.  R.  Watson,  Beaufort  West;  Dr.  E.  Reuning 
and  Mr.  Buvhholz,  Swakopmund;  Miss  Santa  J.  Smuts,  Irene  and 
Pretoria;  Dr.  E.  P.  Phillips,  Division  of  Botany,  Pretoria;  and  Mrs. 
A.  B.  Emery,  Messina.  It  is  a  pleasure  to  acknowledge  the  interest 
of  P.  S.  M.  Arbuthnot,  esq.,  Wynberg,  as  whose  motor  guest  the 
writer  enjoyed  a  special  trip  along  the  south  coast  to  Mossel  Bay  and 
George,  and  to  Oudtshoorn,  in  the  Little  Karroo.  Miss  Michell, 
Miss  Stevens,  and  Professor  D.  Thoday,  University  of  Cape  Town,  were 
helpful  in  many  ways,  especially  in  assisting  the  writer  to  become 
better  acquainted  with  the  vegetation  of  Namaqualand  through  the 
labors  of  the  late  Professor  Pearson,  some  of  the  fruits  of  which  are 
deposited  at  the  university.  Col.  Dr.  and  Mrs.  Buist,  Matjesfontein, 
placed  their  excellent  Karroo  garden  at  the  disposal  of  the  writer  and 
were  helpful  in  many  other  ways.  Through  Mr.  C.  Steward,  Chief 
Meteorologist,  Department  of  Irrigation,  data  on  the  rainfall  of  the 
Union  were  obtained.  Mrs.  L.  Bolus,  Bolus  Herbarium,  Cape  Town, 
determined  a  portion  of  the  plants  which  were  studied  in  the  field,  and 
the  balance  were  determined  by  Dr.  R.  Marloth,  Cape  Town.  Two 
special  acknowledgments  must  be  made:  Dr.  Marloth,  who  has 
especial  knowledge  of  the  plants  of  the  Karroos,  generously  advised 
and  assisted  in  very  many  ways.  Botanical  excursions  to  Table 
Mountain  and  to  the  Central  Karroo  were  made  under  his  guidance. 
And,  finally,  Dr.  I.  B.  Pole  Evans,  Chief,  Department  of  Botany  and 
Director  of  the  Botanical  Survey,  supported  the  work  in  a  whole- 
hearted  fashion,  and  lent  his  aid  in  too  many  ways  to  give  in  detail. 
An  eventful  and  most  useful  and  interesting  motor  trip  was  enjoyed 
in  his  company  to  the  Low  Veld,  Messina,  and  it  was  largely  because 
of  the  interest  of  Dr.  Pole  Evans  that  the  writer  enjoyed  the  courtesy 
of  transportation  on  the  railways  of  the  Union,  without  which  his 
travels  would  necessarily  have  been  much  less  and  his  results  corre¬ 
spondingly  cut  down.  Dr.  Pole  Evans  also  actively  supported  the 
atmometry  begun  by  the  writer  and  which  was  later  taken  over 
by  the  Botanical  Survey. 

The  itinerary  included,  in  addition  to  the  motor  trips  as  above 
mentioned,  journey  by  railway  across  the  Central  or  Great  Karroo 
to  De  Aar,  thence  through  the  Protectorate  of  Southwest  Africa 
to  Swakopmund,  returning  to  Cape  Town.  Later  Pretoria  was 
visited,  and  in  late  spring  Pietermaritzberg  and  Durban.  The  months 
of  August,  September,  and  October  were  spent  at  Beaufort  West  and 
Matjesfontein,  and  at  certain  stations  between,  when  more  intensive 
studies  on  the  plants,  with  especial  regard  to  their  transpiring  power, 
were  carried  on.  The  seasons  of  the  visit,  therefore,  included  winter 
and  spring. 


10 


FEATURES  OF  THE  VEGETATION  OF  THE 


LIST  OF  SPECIES 

Acanthosicyos  horrida  Welw. 

Adansonia  digitata  L. 

Adenia  schlechteri  Harm. 

Aloe  schlechteri  Schonl. 
striata  Haw. 
variegata  L. 

Anacampseros  papyracea  E.  Mey. 

Antizoma  capensis  Thunb. 

Aptosimum  indivisum  Burck 
Arthraerua  leubnitzii  Schinz. 

Asclepias  filiformis  (?). 

Asparagus  capensis  L. 

stipulaceus  Lam. 
striatus  Thunb. 

Aster  filifolius  Vent. 

Bauhinia  marlothii  Engler. 

Berkheya  obovata  (Thunb.)  Willd. 

Bulbine  rostrata  (Jacq.)  Willd. 

Cadaba  juncea  (L.)  Benth.-Hook. 

Carissa  ferox  (?). 

Celosia  spathulifolia  Engler. 

Chaenostoma  sp. 

Chrysocoma  tenuifolia  Berg. 

Cotyledon  coruscans  Haw. 

decussata  Sims, 
hemisphaerica  L. 
mamillaris  L. 
orbiculata  L. 
paniculata  L.  f. 
reticulata  Th. 

Crassula  columnaris  L. 

lycopodioides  L. 
perfossa  Lam. 
pyramidalis  L. 
quadrangularis  Schonl. 
tetragona  L. 

Cussonia  spicata  Thunb. 

Dicoma  diacanthoides  Less. 

Elytropappus  rhinocerotis  (L.  f.)  Less. 
Eriocephalus  glaber  Thunb. 

Euclea  undulata  Thunb. 

Euryops  lateriflorus  Less. 

tenuissimus  Less. 

Euphorbia  cooperi  N.  E.  Br. 

eustacei  N.  E.  Br. 
mauritanica  L. 
multiceps  Berger, 
mundii  N.  E.  Brown, 
stellaespina  Haw. 
stolonifera  Marl. 

Galenia  africana  L. 

Garuleum  bipinnatum  Less. 

Gazania  pinnata  (Thunb.)  Less. 

Geigeria  passerinoides  (L’Her.)  Harv. 
Gnaphalium  sp.  (?) 

Grewia  cana  Sond. 

Gymnosporia  buxifolia  (L  )  Szysz. 
Helichrysum  ericifolium  Less. 

Hermannia  candicans  Ait. 

Hyobanche  glabrata  (?). 

Indigofera  sp.  (?). 

Kleinia  articulata  Haw. 
radieans  DC. 


AND  GENERA. 

Lebeckia  psiloloba  Walp. 

Loranthus  glaucus  Thunb. 

Lycium  spp.  (?). 

Mesembryanthemum  anatomicum  Haw. 

angulatum  Thunb. 
bolusii  Hook.  f. 
brevifolium  Ait. 
calamiforme  L. 
croceum  Jacq. 
crystallinum  L. 
densum  Haw. 
floribundum  Haw. 
haworthii  Don. 
junceum  Haw. 
magnipunctatum 
Haw. 

nobile  Haw. 
pygmaeum  Haw. 
quadrifidum  Haw. 
spinosum  L. 
splendens  L. 
uncinatum  Mill, 
uniflorum  (n.  s.,  Mrs. 

Bolus,  not  pub.), 
viride  Haw. 

(n.  s.  ?). 

Monechma  sp.  (?). 

Nemesia  sp.  (?). 

Osteospermum  sp.  (?). 

Othonna  pavonia  E.  Mey. 

Pachypodium  bispinosum  (L.  f.)  DC. 

Pelargonium  alternans  Wendl. 

crithmifolium  Sm. 

Peliostomum  sp.  (?). 

Pentzia  virgata  Less. 

Pteronia  flexicaulis  L.  f. 

glomerata  L.  f. 
incana  Less, 
pallens  L.  f. 

Protea  neriifolia  R.  Br. 

Relhania  squarrosa  (L.)  L’Her. 

Rhus  lancea  L. 

viminalis  Vahl. 

Rhus  sp.  (?). 

Royena  pallens  Thunb. 

Salsola  aphylla  L.  f. 

Selago  sp.  (?). 

Senecio  cotyledonis  DC. 
longifolius  L. 

Sesamnothamnus  lugardii  (?). 

Stachys  sp.  (?). 

Stapelia  pillansii  N.  E.  Br. 

Stoebe  sp.  (?). 

Sutherlandia  frutescens  R.  Br. 

Tetragonia  sp.  (?). 

Thesium  horridum  Pilger. 
spinosum  F.  f. 

Tripteris  sinuata  DC. 

Ursinia  sp.  (?). 

Viscum  rotundifolium  L.  f. 

Welwitschia  mirabilis  Hook. 

Zygophyllum  stapfii  Schinz. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


11 


GENERAL  FEATURES  OF  THE  CLIMATE  OF 

SOUTHERN  AFRICA. 

I 

The  leading  features  of  the  climatic  environment  of  plants  of  arid 
regions  can  be  said  to  lie  in  the  temperature  and  its  variation,  in  the 
intensity  of  light,  and  in  the  amount  and  seasonal  distribution  of  the 
rainfall,  together  with  certain  other  related  factors,  including  the 
frequently  low  relative  humidity.  A  change  in  one  of  these,  par¬ 
ticularly  in  the  amount  or  the  season  of  the  rains,  very  profoundly 
modifies  the  rest,  with  consequent  serious  alteration  in  the  environic 
complex  as  a  whole.  Thus,  when  the  rainfall  is  in  the  cool  season 
only,  when  the  temperature  is  low,  the  humidity  high,  and  the  light 
values  low,  the  aridity  of  the  drought  period  is  accentuated  by  the  fact 
of  high  temperatures,  low  relative  humidity,  and  high  light  values. 
When,  on  the  other  hand,  the  rainy  period  coincides  with  the  warm 
season,  the  period  of  drought  is  not  so  markedly  arid,  for  the  reason 
that  the  temperature  at  the  time  is  relatively  low,  with  correspondingly 
high  relative  humidity  and  relatively  low  light  intensity.  Accord¬ 
ingly,  it  is  not  difficult  to  understand  why,  in  regions  of  winter  rainfall, 
the  perennials  should  have  pronounced  xerophytic  characteristics, 
even  if  the  amount  of  rain  be  considerable.  In  regions  with  equal 
precipitation,  but  occurring  the  one  in  the  cool  and  the  other  in  the 
warm  season,  it  appears  to  be  the  rule  that  the  former  is  the  more  arid. 
In  certain  arid  and  semi-arid  regions  of  the  world  both  types  of  rain¬ 
fall  occur.  In  such  event,  if  the  rains  of  summer  and  of  winter  are 
fairly  large,  as  in  southern  Arizona,  the  vegetation  is  correspondingly 
abundant,  but  in  case  the  periodic  rains  are  uncertain,  both  as  to 
amount  and  season,  the  arid  conditions  may  be  intense.  The  latter 
obtains  in  central  Australia  and  in  a  portion  of  South  Africa,  especially 
in  the  Karroos.  A  zone,  in  which  lie  the  Karroos,  extending  north¬ 
westerly  from  Algoa  Bay,  separates  the  region  to  the  west  with  rains 
in  winter  from  the  balance  of  the  country  with  summer  rains.  In 
this  intermediate  belt  the  precipitation  may  tend  toward  the  one  or 
the  other  type,  and  vary  from  season  to  season  in  this  regard.  It  also 
may  be  of  small  amount  or  may  wholly  fail.  It  is  this  zone,  including 
Namaqualand  to  the  west,  which  constitutes  the  most  arid  portion 
of  the  subcontinent. 

Southern  Africa  has  a  mild  temperate  climate.  In  the  interior 
the  summers  are  hot  and  the  winters  are  cool,  with  heavy  frosts  and 
snow  in  the  mountains. 

The  rainfall  is  mainly  periodic.  A  line  extending  northwest  from 
Algoa  Bay  separates  the  regions  on  the  east,  with  rains  mainly  in 
summer,  from  those  on  the  west,  mainly  with  winter  rains  (fig.  6,  p.  22). 
In  the  west-central  and  extreme  western  portions  the  rainfall  may 
be  small  in  amount.  Here  are  the  Karroos,  the  Kalahari,  and  Nama- 


12 


FEATURES  OF  THE  VEGETATION  OF  THE 


qualand.  But  in  the  extreme  southwest,  and  in  the  south,  east,  and 
east-central  portions  of  the  subcontinent  the  rainfall  is  plentiful 
and  the  vegetation  is  varied  and  may  be  abundant. 

The  physical  factors  which  appear  to  mainly  control  the  climate 
are  latitude,  topography,  proximity  to  the  oceans,  and  relation  to 
regions  of  permanent  low  atmospheric  pressure  both  to  the  east  and 
to  the  west. 

The  main  facts  in  regard  to  the  physiographical  characteristics  are 
succintly  stated  by  Cox,1  as  follows: 

“ There  are  essentially  four  elevated  plateaux;  the  Coast  Flats,  with  an 
elevation  of  500  to  600  ft.,  and  a  variation  in  width  from  thirty  miles  in 
Southwest  Africa  to  three  miles  or  even  less  in  the  southeast  of  the  Cape 
Province;  the  Little  Karroo,  a  narrow  stretch  of  from  fifteen  to  twenty  miles, 
with  an  elevation  of  about  1,500  ft.;  the  Great  Karroo  at  an  altitude  of  from 
2,000  to  3,000  ft.,  and  the  Northern  Karroo  with  an  elevation  of  4,000  ft., 
rising  to  6,000  ft.  in  the  eastern  portions.  These  plateaux  are  separated  by 
steep  escarpments,  rising  a  considerable  height  above  them.”  (Fig.  1.) 

Variations  in  physiography  affect  the  climate  mainly  along  two 
lines.  On  the  one  hand,  differences  in  altitude  tend  to  overcome  dif¬ 
ferences  in  latitude,  and  on  the  other  they  directly  affect  rainfall. 
Referring  to  the  former,  Cox  presents  data  in  which  it  appears  that 
although  the  difference  in  latitude  between  Pretoria  and  Mossel  Bay, 
for  example,  is  nearly  9°,  the  mean  annual  temperature  is  almost  the 
same.  Pretoria  is  approximately  4,000  feet  greater  in  altitude  than 
Mossel  Bay.  So  far  as  the  effect  of  mountains  on  rainfall  is  concerned, 
it  will  suffice  to  remark  here  that  aside  from  the  increase  with  altitude, 
as  in  mountains,  mountains  also  serve  to  remove  moisture  from  the 
passing  air-currents,  so  that  they  may  not  deposit  moisture  on  adjacent 
lower  lands.  Such  is  the  condition  in  the  Karroos,  which  are  sep¬ 
arated  from  each  other  and  from  the  coast  by  mountain  chains  where 
the  rainfall  decreases  in  general  from  the  coast  inland,  as  from  the 
southern  to  the  northern  Karroos. 

The  relatively  small  land  area  with  long  shore-line  and  with  immense 
bodies  of  water  on  three  sides,  are  important  factors  in  the  shaping  of 
the  southern  African  climate.  On  the  west  is  the  cold  Benguela  cur¬ 
rent  of  the  southern  Atlantic,  and  on  the  south  and  east  is  the  warm 
Mozambique  current.  The  difference  in  temperature  at  the  same 
latitude  between  the  east  and  the  west  coasts  is  striking.  Thus2  at 
latitude  30°,  which  is  about  that  of  Durban,  the  mean  monthly  temp¬ 
eratures  of  the  surface  water  somewhat  off-shore  for  the  months 
given  are  as  shown  at  top  of  next  page. 

1  A  guide  to  botanical  survey  work.  Botanical  Survey  of  South  Africa,  Mem.  No.  4,  p.  27, 
1922. 

2  Publication  of  the  British  Meteorological  Office,  Official  No.  59. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


13 


Feb. 

May. 

Aug. 

Nov. 

°  F. 

0  F. 

°  F. 

0  F. 

Atlantic  Ocean . 

66 

65 

60 

62 

Indian  Ocean . 

75 

74 

69 

72 

Such  differences  in  the  temperature  of  the  sea  directly  affect  the 
temperature  of  coastwise  stations.  Thus,  Port  Nolloth,  on  the  At¬ 
lantic,  lat.  29°  16',  has  a  mean  annual  temperature  of  57.5°  F.,  and 
Port  St.  Johns,  Indian  Ocean,  lat.  31°  38',  has  a  mean  annual  temper¬ 
ature  of  66.9°  F.  (Cox,  Z.  c.). 

Not  only  do  the  ocean  currents  thus  directly  affect  the  temperature 
of  adjacent  points  on  shore,  but  in  that  they  vary  in  capacity  of  de¬ 
livering  moisture  to  the  air  they  are  important  in  modifying  the  rainfall 
conditions  as  well;  and  in  connection  with  the  prevailing  winds  and 
other  factors  they  may  ultimately  influence  the  rainfall  in  places  as 
remote  as,  for  example,  the  highlands  of  the  Protectorate  of  Southwest 
Africa,  800  miles  or  more  from  the  Indian  Ocean. 

The  part  played  by  high-pressure  areas  off  the  east  and  west  coasts 
is  outlined  by  Cox  (Z.  c.)  as  follows: 

“The  seasonal  distribution  of  rain  is  related  to  the  movement  and  action 
of  the  permanent  anti-cyclones  which  lie  off  the  west  coast  of  the  Cape 
Province  and  off  the  east  coast  between  the  Cape  Province  and  Australia. 
This  belt  of  high  pressure  migrates  northwards  and  southwards  with  the 
sun,  and  in  addition  the  centres  or  cores  have  a  lateral  displacement  from 
month  to  month.  During  April  and  May  that  to  the  east  of  the  Cape 
Province  moves  westward  to  the  African  coasts,  while  that  on  the  west  coast 
moves  eastwards.  At  the  same  time  an  important  secondary  core  appears 
over  the  land,  where  barometric  pressure  increases  until  June  or  July.  The 
movement  northwards  of  the  anti-cyclonic  belt  brings  the  west  and  southwest 
coastal  regions  of  the  Cape  Province  under  the  influence  of  A-shaped  depres¬ 
sions  connected  with  the  cyclonic  system  to  the  south;  and  it  is  the  westerly 
winds  associated  with  the  rear  of  these  depressions  which  are  the  rain-bearers 
for  the  west  and  southwest  coastal  districts  of  the  Cape  Province,  where  over 
75  per  cent  of  the  annual  precipitation  occurs  during  the  winter.  As  will 
be  seen,  the  area  thus  watered  is  not  extensive.  Originating  in  the  cold 
parts  of  the  Atlantic  the  capacity  of  the  westerly  winds  for  moisture  is  small, 
and  after  condensation,  forced  by  the  elevated  ground  which  forms  the  western 
boundaries  of  the  plateaux,  they  soon  cease  to  act  as  rain-bearers. 

“In  September  and  October  the  high  pressure  moves  off  the  land,  merging 
into  the  South  Indian  anti-cyclone,  which  then  returns  eastwards  to  its  sum¬ 
mer  position  just  off  the  west  coast  of  Australia,  and  the  South  Atlantic  anti¬ 
cyclone  which  lies  a  short  distance  from  the  west  coast  of  the  Cape  Province. 
The  north-easterly  and  easterly  winds  associated  with  the  former  introduce 
the  moisture  which  is  deposited  over  the  greater  part  of  the  Union  during 
the  summer  months.  These  winds  when  leaving  the  Indian  Ocean  are  warm 
and  their  capacity  for  moisture  is  great;  and  although  they  deposit  a  consid¬ 
erable  amount  of  moisture  in  ascending  the  plateaux  .  .  .  .,  and  so  decrease 
their  absolute  humidity,  they  still  reach  the  interior  with  a  comparatively 
high  humidity.” 


14 


FEATURES  OF  THE  VEGETATION  OF  THE 


TEMPERATURE. 

As  to  the  temperature  of  southern  Africa  in  general,  according  to 
Knox,  from  whose  work  much  of  the  information  on  the  climate  of 
South  Africa  here  used  is  freely  drawn,1  there  is  a  gradual  increase  on 
any  parallel  of  latitude  from  the  west  to  the  east,  and  along  the  west 
coast  from  the  north  to  the  south,  and  along  the  east  coast  from  the 
south  to  the  north.  Thus,  taking  Port  Nolloth,  O’okiep,  Kimberley, 
and  Durban,  all  of  which  are  not  far  from  the  same  latitude,  the  fol¬ 
lowing  are  the  mean  annual  temperatures,  namely:  57.5°,  63°,  64°, 

1 


Fig.  1. — Midwinter  and  midsummer  mean  isotherms.  The  shaded  contours  delimit  ap¬ 
proximately  the  4000-foot  level. 

and  79.8°  F.  The  mean  annual  temperature  at  Walfisch  Bay  is  59.5°  F. 
and  at  Table  Bay  61.3°  F.  The  mean  annual  temperature  for  South 
Africa  is  nowhere  far  from  62°  F.,  the  mean  for  Cape  Town.  It  is 
62.3°  F.  at  Grahamstown,  64.2°  at  Graaf  Reinet,  63.8°  at  Pretoria,  and 
64.3°  at  Pietersburg.  At  Pietermaritzburg  in  Natal  it  is  66.3°  F. 
A  relatively  low  mean  annual  temperature  is  to  be  found  at  Port 
Nolloth,  which  is  57.6°  F.  There  is,  however,  a  considerable  dif¬ 
ference  in  various  regions  of  South  Africa  between  the  absolute  maxima 
and  minima,  the  mean  maxima  and  minima,  and  the  daily  range,  as 


1  The  climate  of  the  continent  of  Africa.  Alexander  Knox,  Cambridge,  1911. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


15 


would  be  expected,  all  of  which  are  especially  well  marked  in  the 
more  arid  regions.  The  mean  extremes  of  the  Cape  Peninsula  are 
80°  and  47°  F.  The  mean  for  the  warm  season  at  the  Cape  is  69.7° 
and  for  the  cold  season  is  58.4°  F.  For  the  southern  Karroo  the  means 
of  the  two  seasons  are  75.5°  and  51°  F.  For  the  central  and  western 
portions  of  the  Central  Karroo,  in  which  are  included  Beaufort  West 
and  Prince  Albert  Road,  as  well  as  Matjesfontein  and  Laingsburg, 
the  means  are  58°  to  59.6°  F.  for  the  warm  season  and  50°  F. 
for  the  cold  season.  In  the  east  Central  Karroo  the  mean  for  the 
warm  season  is  78.7°  F.  In  this  division  is  included  Aberdeen  and 
Graaf  Reinet.  In  the  northern,  or  Upper  Karroo,  which  extends  from 


Table  1. — Temperature,  in  degrees  Fahrenheit .° 


Mean 

tem¬ 

perature. 

Mean. 

Absolute. 

Mean  of 
absolute 
yearly. 

Mean 

daily 

range. 

Max. 

Min. 

Max. 

Min. 

Max. 

Min. 

Table  Bay . 

61.3 

80.0 

47.0 

101.3 

34.1 

101.3 

34.1 

10.6 

Pietermaritzburg . 

66.3 

79.7 

52.9 

109.0 

27.0 

106.0 

32.0 

26.8 

Bloemfontein . 

61.9 

75.4 

48.4 

101.7 

22.0 

26.5 

Johannesburg . 

61.2 

74.4 

47.9 

96.0 

21.0 

26.5 

Graaf  Reinet . 

64.2 

78.7 

49.6 

108.0 

29.7 

29.1 

Kimberley . 

65.8 

81.5 

50.2 

104.3 

21.5 

31.3 

Windhoek . 

66.5 

79.2 

53.9 

98.3 

26.5 

25.3 

Swakopmund . 

59.5 

68.1 

53.7 

105.1 

36.5 

14.4 

Walfisch  Bay . 

59.5 

65.9 

53.2 

94.8 

34.9 

12.7 

°  Compiled  from  The  climate  of  the  continent  of  Africa.  Alexander  Knox.  Cambridge,  1911. 


a  little  to  the  east  of  Clan  william  eastward,  the  mean  temperature  of 
the  warm  season  is  70.7°  to  72.3°  F.  and  that  of  the  cold  season  is  42.3° 
to  50°  F.  The  means  of  the  northern  border,  including  the  eastern 
parts  of  Namaqualand  and  Clanwilliam  divisions,  in  which  is  Upington, 
are  78.5°  and  51°  F.  As  to  absolute  maximum  temperatures,  there  have 
been  recorded  in  the  Cape  Peninsula  101.3°;  the  Southern  Karroo, 
112°;  West  Central  Karroo,  1 16.06°  ;*  East  Central  Karroo,  108°; 
Northern  Karroo,  110°,  and  the  northern  border,  112°  F. 

In  Southwest  Africa,  according  to  Knox,  the  summers  are  hot,  which 
applies  to  stations  not  on  the  seaboard,  and  the  winters  moderate, 
with  not  infrequent  frosts  in  the  interior.  There  is  relatively  wide 
range  in  daily  and  annual  temperatures.  At  Windhoek,  altitude  5,428 
feet,  the  mean  temperature  of  the  warm  season  is  73.6°  and  of  the  cold 
season  is  57.3°  F.  The  mean  annual  temperature  is  66.2°.  The  abso¬ 
lute  maximum  at  Windhoek  is  98.8°  and  the  absolute  minimum  is 
26.5°  F.  One  of  the  characteristic  climatic  features  of  Windhoek  is 
that  the  mean  temperature,  the  mean  maximum,  and  mean  mini- 


1  Prince  Albert,  1883.  Das  Klima  dea  aussertropischen  Sudafrika.  Dove,  p.  66,  1888. 


16 


FEATURES  OF  THE  VEGETATION  OF  THE 


mum  temperature,  are  very  regular  for  the  various  months  from  year 
to  year.  Thus  in  course  of  four  consecutive  years,  in  March,  the 
mean  minimum  temperature  ranged  between  57.7°  and  58.6°  F.,  and 
the  mean  maximum  temperature  for  the  same  years  ranged  between 
72.9°  and  74.3°  F.  At  Swakopmund  the  mean  annual  temperature 
is  59.5°;  in  the  warm  season  the  mean  temperature  is  62.7°,  and  in  the 
cold  season  it  is  56.6°.  The  absolute  maximum  at  Swakopmund  is 
105.1°  and  the  absolute  minimum  is  36.5°  F.  At  Swakopmund  the 
presence  of  the  ocean  and  of  the  summer  fog  are  important  controls 
of  the  temperature. 

RAINFALL  IN  SOUTHERN  AFRICA. 

GENERAL  CONDITIONS. 

Where  local  circumstances  do  not  intervene  to  prevent,  the  rainfall 
increases  from  the  west  to  the  east,  and  in  a  less  marked  manner  and 
less  constant,  according  to  Knox,  from  the  south  to  the  north.  Knox 
illustrates  these  conditions  by  citing  the  precipitation  at  Keetmans- 
hoop,  Southwest  Africa,  Vryburg,  Bechuanaland,  and  Lourenco 
Marques,  Portuguese  East  Africa,  where  the  mean  annual  rainfall  is 
6.38,  22.38,  and  28.23  inches,  respectively,  and  also  at  a  series  of 
stations  situated  somewhat  farther  to  the  north.  So  far  as  the  increase 
of  the  rainfall  from  the  south  to  the  north  is  concerned,  Knox  refers 
to  Graaf  Reinet,  Central  Karroo,  mean  annual  rainfall  15.29  inches, 
Kimberley,  in  the  northern  border,  18.17  inches,  and  Vryburg,  22.38 
inches.  The  altitude  of  Graaf  Reinet  is  2,463  feet,  that  of  Vryburg 
3,890  feet,  and  that  of  Kimberley  4,012  feet.  The  relief  of  South 
Africa  is  important  in  modifying  the  general  rainfall.  Thus,  the  rain¬ 
fall  of  the  table-land  of  Southwest  Africa,  in  the  neighborhood  of 
Windhoek,  is  heavier  than  that  of  the  Kalahari,  with  lower  altitude, 
to  the  east,  and  the  rainfall  of  the  extreme  southwest  is  relatively 
heavy,  being,  according  to  Knox,  greater  than  in  any  other  of  the 
Cape  districts.  As  before  remarked,  regions  of  high  altitude  have 
relatively  heavy  precipitation,  which  is  of  some  importance  in  operat¬ 
ing  to  modify,  in  certain  particulars,  features  of  the  climate  in  the 
rain  shadow  of  such  highlands,  or  at  any  rate  in  contiguous  regions  of 
lower  altitude.  Under  proper  conditions  of  air-currents  the  relative 
humidity,  and  hence  the  evaporation,  of  such  contiguous  regions  is 
probably  directly  and  markedly  affected  by  the  regions  of  high  rainfall. 
Although  not  sufficient  study  has  been  given  this  phase  of  the  general 
subject  (which  is  discussed  elsewhere)  to  give  satisfactory  details, 
observations,  nevertheless,  appear  to  verify  the  statement.  Another 
climatic  factor,  which  such  contiguity  of  regions  would  influence,  is 
temperature,  which  need  not  be  referred  to  further. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


17 


The  general  terrace  formation  of  southern  Africa,  the  fact  of  im¬ 
portant  mountain  masses  and  chains  which  separate  regions  of  unlike 
altitude,  and  which  in  a  general  way  run  parallel  to  the  coast,  are  of 
moment  in  providing  local  conditions  which  serve  to  modify  those 
generally  obtaining.  In  the  eastern  portion  of  southern  Africa,  the 
coastal  plateau  is  relatively  narrow,  but  in  the  southern  and  south¬ 
western  portions  it  is  fairly  wide,  and  several  ranges  of  mountains 
intervene  between  the  coast  and  the  height  of  land  relatively  far 
inland.  Thus,  going  northward  from  Knysna,  one  ascends  and  passes 
through  the  Outeniqua  Mountains  to  the  Southern  Karroo,  one  crosses 
the  Southern  Karroo,  ascends  and  passes  through  the  Groote  Zwatre 
Bergen,  to  the  Central  Karroo ;  and  one  crosses  the  Central  Karroo  to 
and  through  the  Nieuweveld  range  to  the  Northern  Karroo.  The 
Southern  or  Little  Karroo  is  between  1,000  and  2,000  feet,  the  Central 
Karroo  between  2,000  and  3,000  feet,  and  the  Northern  Karroo  4,000 
feet  or  more  in  altitude.  The  mountain  ranges  which  separate  the 
terraces  have  an  altitude  of  from  3,000  to  6,000  feet.  A  very  large 
proportion  of  southern  Africa  has  an  altitude  of  4,000  feet,  or  more. 
It  can  be  seen  that  the  peculiar  relief  of  southern  Africa  must  be,  as 
it  in  fact  is,  of  importance  in  shaping  the  climate  of  this  portion  of  the 
continent.  It  operates  not  only  to  modify  the  temperature,  but  the 


Fig.  2. — Average  annual  rainfall.  In  part  from  Mem.  4,  Bot.  Sur.  So.  Africa,  1922. 


18 


FEATURES  OF  THE  VEGETATION  OF  THE 


rainfall  as  well.  The  higher  altitudes  serve  to  condense  the  moisture 
of  moisture-laden  winds  coming  from  the  east  coast  and  from  the  west 
coast,  and  the  precipitation  in  such  areas  may  be  large.  But,  on  the 
other  hand,  more  lowly  lying  but  contiguous  regions,  in  the  lee  of 
the  highlands,  may  for  this  or  other  reasons  have  a  small,  or  relatively 
small,  annual  precipitation. 

The  wettest  station  so  far  reported  in  South  Africa  is  Maclear’s 
Beacon,  Table  Mountain,  where  the  average  for  seven  years,  1894  to 
1900,  was  86.8  inches  annually.1 


Table  2. — Seasonal  distribution  of  rainfall .a 


No.  of 
years. 

Average 
annual 
rainfall, 
in  inches. 

Percentage  of  seasonal  rainfall,  with  amount,  in  inches,  in 

parenthesis. 

Dec.-Feb. 

Mar.-May. 

June- Aug. 

Sept.-Nov. 

Cape  Town . 

59 

25.09 

7 

(  1-7) 

26 

(6.1  ) 

48 

(10.1  ) 

19 

(  4.5) 

Grahamstown . 

43 

26.36 

26 

(  7.9) 

26 

(7.9  ) 

11 

(  3.3  ) 

37 

(10.9) 

Pietermaritzburg  . . . 

26 

37.13 

42 

(14.4) 

21 

(7.5  ) 

5 

(  1-8  ) 

31 

(10.7) 

Ladysmith . 

39 

14.08 

22 

(  3.2) 

30 

(4.4  ) 

25 

(  3.6  ) 

23 

(  3.3) 

Montagu . 

39 

12.8 

13 

(  1.6) 

30 

(3.8  ) 

34 

(  4.4  ) 

23 

(  3.0) 

Oudtshoorn . 

42 

9.45 

20 

(  1-7) 

29 

(2.4  ) 

19 

(  1-6  ) 

32 

(  2.7) 

Uniondale . 

42 

13.42 

19 

(  2.6) 

29 

(0.4  ) 

25 

(  3.3  ) 

26 

(  3.5) 

Aberdeen . 

39 

12.19 

37 

(  4.4) 

30 

(3.6  ) 

9 

(  i.o  ) 

24 

(  2.8) 

Beaufort  West . 

43 

9.56 

32 

(  2.9) 

36 

(3.4  ) 

10 

(  0.9  ) 

22 

(  0.2) 

Graaf  Reinet . 

40 

13.87 

30 

(  4.5) 

31 

(4.7  ) 

9 

(  1-3  ) 

30 

(  4.5) 

Laingsburg . 

18 

4.47 

20 

(  0.8) 

36 

(0.6  ) 

27 

(  1-2  ) 

17 

(  0.7) 

Matjesfontein . 

33 

6.5 

17 

(  1.1) 

33 

(2.1  ) 

24 

(  1-6  ) 

24 

(  1.5) 

O’okiep . 

26 

6.73 

9 

(  0.6) 

31 

(2.0  ) 

41 

(  2.6  ) 

19 

(  1.2) 

Upington . 

28 

10.81 

38 

(  4.1) 

33 

(3.57) 

2 

(  0.19) 

17 

(  1.9) 

Pretoria . 

18 

26.9 

55 

(14.8) 

17 

(4.58) 

2 

(  0.45) 

27 

(  7.1) 

Petersburg . 

17 

20.6 

58 

(12.0) 

17 

(3.5  ) 

2 

(  0.5  ) 

22 

(  4.6) 

Messina . 

11 

14.2 

69 

(  9-8) 

12 

(1.7  ) 

1 

(  0.2  ) 

18 

(  2.5) 

Warmbad . 

6 

5.6 

36 

(  2.0) 

45 

(2.5  ) 

8 

(  0.5  ) 

11 

(  0.6) 

Luederitz  Bay . 

6 

2.8 

32 

(  0.9) 

17 

(0.5  ) 

43 

(  1-2  ) 

7 

(  0.2) 

Keetmanshoop . 

6 

6.3 

47 

(  3.0) 

35 

(2.2  ) 

2 

(  0.1  ) 

16 

(  0.1) 

Bethany . 

6 

5.3 

51 

(  2.7) 

34 

(1.8  ) 

2 

(  0.1  ) 

13 

(  0.7) 

Gibeon . . 

6 

7.5 

55 

(  4.1) 

34 

(2.6  ) 

1 

(  0.1  ) 

9 

(  0.7) 

Swakopmund . 

6 

0.98 

71 

(  0.7) 

18 

(0.18) 

0 

10 

(  0.1) 

Windhoek . 

14 

15.08 

54 

(  8.2) 

37 

(5.2  ) 

2 

(  0.26) 

9 

(  1.3) 

Gobabis . 

6 

15.4 

59 

(  9.2) 

29 

(4.5  ) 

0 

11 

(  1.7) 

Karibib . 

6 

7.7 

65 

(  5.0) 

35 

(2.7  ) 

0 

0 

Grootfontein . 

6 

23.13 

60 

(13.8) 

28 

(6.7  ) 

0 

11 

(  2.5) 

a  The  data  are  mainly  from  the  Meteorological  Office,  Pretoria,  and  from  Knox,  The  climate  of 
the  continent  of  Africa,  Cambridge,  1911.  Where  there  are  differences  between  the  “average” 
rainfall  and  the  sum  of  the  seasonal  rains,  the  reason  lies  in  the  differences  in  the  data  on  which 
they  are  based. 

It  will  be  of  interest  now  to  review  in  some  detail  some  of  the  leading 
characteristics  of  the  rainfall  of  representative  stations  of  the  Union 
and  of  the  Protectorate  of  Southwest  Africa.  Although  in  doing  so 
attention  will  be  especially  directed  to  regions  of  which  the  study 
principally  deals,  that  is,  those  having  a  small  rainfall,  it  will  neverthe¬ 
less  be  instructive  to  refer  to  the  rainfall  conditions  of  certain  other 


1  Science  in  South  Africa;  The  meteorology  of  South  Africa.  C.  M.  Stewart.  Page  28,  1905. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


19 


regions,  not  necessarily  very  remote,  in  which  the  rainfall  is  relatively 
or  actually  large,  and  which  for  this  reason  have  a  marked  influence 
on  the  climate  of  the  regions  first  referred  to  above,  that  is,  those  in 
which  the  annual  precipitation  is  relatively  or  actually  small. 

It  has  already  appeared,  and  is  generally  known,  that  a  marked 
feature  of  the  rainfall  of  South  Africa  taken  as  a  whole  is  its  periodicity. 
There  are  regions,  however,  in  which  the  periodicity  of  the  rainfall  is 
not  an  especially  prominent  feature,  but  between  the  .Cape  and  Natal, 
not  to  mention  other  sections  of  the  Union,  there  is  a  region  in  which 
the  rainfall  is  fairly  well  distributed  through  the  four  seasons.  Knysna, 
George,  and  Grahamstown  are  located  in  this  intermediate  region. 
The  annual  precipitation,  especially  of  the  two  first  given,  is  uniformly 
distributed,  and  also  that  of  Grahamstown,  but  not,  however,  so  uni¬ 
formly.  The  statistics  of  rainfall  for  Grahamstown  are  at  hand,  and 
will  be  referred  to  as  representing  the  region.  The  precipitation  at 
Grahamstown  averages  26.36  inches,  of  which  about  25  per  cent 
occurs  in  summer,  26  per  cent  in  autumn,  36  per  cent  in  spring,  and 
somewhat  more  than  11  per  cent  in  winter.  The  relative  dependabil¬ 
ity  of  the  rainfall  at  Grahamstown  can  be  further  seen  from  the  accom¬ 
panying  table  on  rainfall  extremes.  Here  it  will  be  noted  that  the 


Fig.  3. — Seasonal  distribution,  in  percentages,  of  rainfall.  Adapted  from  Mem.  4,  Bot. 

Sur.  So.  Africa,  1922. 


20 


FEATURES  OF  THE  VEGETATION  OF  THE 


yearly  extremes  at  Grahamstown  are  42.52  and  17.78  inches,  that  the 
extremes  for  summer,  autumn,  and  spring  are  relatively  small,  and 
that  the  actual  rainfall  for  the  three  seasons  is  relatively  large  in 
amount.  But,  on  the  other  hand,  during  the  winter  the  rainfall  is  not 
only  small,  but  also  it  is  relatively  variable.  The  dependability 
of  the  rainfall  at  Grahamstown  can  be  illustrated  in  yet  another  way. 
Referring  to  table  3,  which  gives  the  periods  having  less  than  0.15 
inch  rainfall  per  month,  it  will  be  seen  that  at  Grahamstown  about 
three  months  every  year  have  rain  of  this  small  amount,  and  that 
the  rainfall  of  spring  and  of  summer  is  always  more  than  0.15  inch 
per  month,  but  that  of  the  total  yearly  precipitation  occurring  in  this 
small  monthly  amount  90  per  cent  is  in  winter.  It  thus  appears 
that  at  Grahamstown,  and  probably  also  in  this  general  region,  the 
rainfall  is  periodic,  but  not  particularly  small,  and  it  does  not  wholly 
fail  in  any  season.  These  features,  as  is  pointed  out  in  another  place, 
are  of  undoubted  importance  in  modifying  to  a  certain  limited  extent 
the  severe  conditions  of  aridity  in  certain  arid  regions,  especially  the 
Karroos,  further  to  the  north. 

When  one  goes  much  to  the  west,  or  to  the  east  of  the  intermediate 
region  just  spoken  of,  the  rain  may  be  found  to  increase,  or  may  not, 
but  in  any  event,  the  periodicity  of  the  rainfall  becomes  an  increasingly 


Fig.  4. — Rainfall  for  January  1920.  Adapted  from  weather  report. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


21 


prominent  characteristic  of  the  climate.  At  Cape  Town  the  average 
rainfall  is  25.09  inches,  of  which  0.17  per  cent  occurs  in  summer, 
25  per  cent  in  autumn,  47  per  cent  in  winter,  and  19  per  cent  in  spring; 
or,  in  other  words,  approximately  four-fifths  of  the  total  average 
rainfall  at  Cape  Town  occurs  in  six  months,  March  to  August,  leaving 
the  balance  of  the  year  with  little  rain.  At  Pietermaritzburg,  on 
the  other  hand,  with  a  rainfall  of  37.13  inches,  about  41  per  cent  occurs 
in  summer,  21  per  cent  in  autumn,  31  per  cent  in  spring,  and  only  5.8 
per  cent  in  winter.  These  two  stations  also  can  be  used  to  illustrate 
other  conditions  attending  the  rain  of  winter  and  drought  of  summer 
on  the  one  hand,  and  the  rain  of  summer  and  drought  of  winter  on 
the  other.  Thus,  the  rainfall  extremes  at  both  Cape  Town  and 
Pietermaritzburg  are  a  characteristic  of  the  climate  of  each.  At 
Cape  Town  the  yearly  extremes  are  33.98  and  17.52  inches,  and  at 


Table  3. — Brought  periods  ( periods  with  less  than  0.15  inch  rainfall  in  a  month ,  or  no 

precipitation ).a 


Length 

of 

record 

in 

years. 

Average 
yearly 
drought 
period 
in  months. 

Seasonal  percentage  of  drought  period. 

Dec.-Feb. 

Mar. -May. 

June- Aug. 

Sep.-Nov. 

Cape  Town . 

59 

0.93 

83.6 

12.7 

0 

3.6 

Grahamstown .... 

43 

0.23 

0 

10.0 

90.0 

0 

Pietermaritzburg . 

26 

1.6 

0 

11.9 

88.09 

0 

Ladysmith . 

39 

1.2 

44.6 

17.0 

10.6 

27.6 

Montagu . 

37 

2.2 

57.3 

18.2 

6.0 

18.2 

Oudtshoorn . 

42 

2.2 

43.1 

18.9 

15.7 

22.1 

Uniondale . 

42 

1.2 

53.8 

17.3 

19.2 

9.6 

Aberdeen . 

39 

3.0 

12.0 

14.9 

51.4 

21.49 

Beaufort  West.  .  . 

43 

3.1 

19.8 

13.9 

40.0 

25.7 

Graaf  Reinet.  .  .  . 

40 

2.1 

16.4 

8.3 

51.7 

22.1 

Laingsburg . 

18 

5.2 

34.0 

17.0 

17.0 

31.9 

Matjesfontein. . . . 

33 

3.9 

35.3 

20.0 

15.3 

29.2 

O’okiep . 

26 

5.9 

42.5 

17.4 

9.6 

30.0 

Upington . 

28 

5.2 

16.8 

14.2 

44.1 

24.6 

Pretoria . 

18 

3.2 

0 

17.2 

70.0 

12.0 

Pieters  burg . 

17 

3.8 

1.5 

20.0 

63.9 

15.3 

Messina . 

11 

5.1 

0 

22.7 

49.1 

28.0 

Warmbad . 

b  6 

5.9 

10.0 

23.3 

30.0 

36.6 

Luederitz  Bay .. .  . 

b  6 

7.7 

15.7 

28.9 

21.0 

34.2 

Keetmasnhoop .  . . 

b  6 

5.5 

3.1 

18.6 

46.8 

31.2 

Bethany . 

b  6 

6.5 

3.0 

21.2 

45.4 

30.3 

Gibeon . 

b  6 

6.2 

8.1 

2.7 

48.6 

32.4 

Swakopmund .... 

b  6 

9.5 

14.8 

27.6 

31.8 

25.5 

Windhoek . 

6 

5.3 

0 

15.6 

53.1 

31.2 

Gobabis . 

6 

4.8 

0 

20.6 

58.6 

20.6 

Karibib . 

66 

5.9 

0 

20.5 

52.1 

26.5 

Grootfontein . 

6 

4.8 

0 

20.0 

60.0 

20.0 

a  Compiled  from  meteorological  records;  those  for  the  Union  of  South  Africa  furnished  by  the 
Meteorological  Office,  Pretoria,  and  those  for  Southwest  Africa  taken  from  Arbeiten  d.  Farm- 
wirtschaft-Gesellsch.  f.  Siidwest  Afrika,  Bd.  2.  May,  1921. 

6  Records  not  complete;  those  for  Warmbad,  58  months;  for  Luederitz  Bay,  61  months;  for 
Keetmanshoop,  65  months;  for  Gibeon,  71  months;  for  Swakopmund,  58  months;  and  for 
Karibib,  71  months. 


22 


FEATURES  OF  THE  VEGETATION  OF  THE 


Fig.  5. — Rainfall  for  August  1920.  Adapted  from  weather  report. 


Fig.  6.— Minimal  rainfall,  1885-1894.  In  part  after  Marloth,  Das  Kapland.  The  heavy 
line  running  northwest  from  near  Port  Elizabeth  approximately  separates  the  region 
of  summer  rains,  to  the  east,  from  that  of  winter  rains. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


23 


Pietermaritzburg  they  are  54.43  and  25.19  inches.  As  table  4  shows, 
the  seasonal  and  the  monthly  extremes  are  at  each  station  a  marked 
characteristic  of  the  rainfall.  When  we  compare  the  relative  amount 
of  rain  that  occurs  in  monthly  amounts  less  than  0.15  inch,  we  find, 
however,  that  the  average  yearly  total  of  such  small  amount  is  some¬ 
what  less  at  Cape  Town  than  at  the  station  in  Natal,  indicating  that 
there  are  fewer  days  with  a  small  rainfall  at  Cape  Town  than  at 


Table  4. — Rainfall,  in  inches,  compiled  from  meteorological  records. a 


Yearly 

extremes. 

Dec.- 

-Feb. 

Seasonal 

Mar.-May 

extremes. 

June- Aug. 

Sept.- 

-Nov. 

Monthly 

extremes. 

Daily 

max. 

Max. 

Min. 

Max. 

Min. 

Max. 

Min. 

Max. 

Min. 

Max. 

Min. 

Max. 

Min. 

Cape  Town . 

33.98 

17.52 

5.50 

0 

12.45 

1.45 

22.30 

4.38 

7.96 

2.65 

13.27 

0 

3.88 

Grahamstown . 

42.52 

17.78 

2.87 

2.16 

3.12 

2.08 

1.62 

0.67 

4.55 

3.10 

10.69 

0 

7.28 

Pietermaritzburg .... 

54.43 

25.19 

30.98 

9.22 

13.43 

3.18 

7.72 

0 

25.80 

4.67 

13.36 

0 

5.85 

Ladvsmith . 

21.79 

7.67 

5.74 

0.29 

8.97 

0.77 

7.20 

0.84 

6.75 

0.55 

6.36 

0 

4.06 

Montagu . 

22.08 

7.66 

7.37 

0 

8.70 

1.19 

7.6 

0.98 

5.90 

0.69 

7.59 

0 

6.75 

Oudtshoorn . 

21.46 

7.12 

4.00 

0 

7.39 

0.64 

4.7 

0.39 

9.83 

0.67 

3.9 

0 

3.75 

Uniondale . 

28.11 

7.33 

5.35 

0.31 

12.88 

0.95 

8.5 

1.18 

10.75 

0.67 

10.48 

0 

7.67 

Aberdeen . 

19.23 

4.18 

9.70 

1.00 

8.30 

0.84 

4.18 

0.03 

7.50 

0.26 

5.72 

0 

3.92 

Beaufort  West . 

18.26 

3.19 

9.12 

0.43 

9.85 

0.85 

2.5 

0.10 

5.70 

0.23 

9.70 

0 

8.08 

Graaf  Reinet . 

21.46 

5.79 

10.2 

1.32 

9.73 

1.17 

4.73 

0.04 

7.66 

0.39 

5.26 

0 

6.50 

Laingsburg . 

7.10 

1.06 

2.82 

0 

4.36 

.29 

2.19 

0 

2.00 

0 

3.18 

0 

1.75 

Matjesfontein . 

10.38 

2.16 

4.90 

0 

6.13 

0 

5.17 

0 

4.73 

0 

3.30 

0 

2.06 

O’okiep . 

11.7 

3.46 

2.55 

0 

6.04 

0.63 

6.17 

0.70 

3.34 

0.11 

3.49 

0 

2.04 

Upington . 

15.52 

1.33 

6.99 

0.09 

9.8 

0.51 

1.59 

0 

5.6 

0 

7.4 

0 

2.52 

Pretoria . 

55.90 

16.68 

35.82 

6.78 

11.11 

2.09 

4.05 

0 

10.57 

4.81 

22.12 

0 

5.33 

Pietersburg . 

31.53 

10.47 

20.77 

5.51 

6.77 

0.56 

2.71 

0 

8.73 

1.47 

12.18 

0 

4.30 

Messina . 

22.05 

9.40 

7.63 

2.71 

4.29 

0.29 

1.20 

0 

6.03 

0.39 

8.63 

0 

5.28 

°  Supplied  by  the  Meteorological  Office,  Pretoria. 


Pietermaritzburg.  Such,  as  a  matter  of  fact,  is  the  case.  From 
data  furnished  by  the  Meteorological  Office,  it  appears  that  at  Cape 
Town  there  are  on  an  average  36.3  days  of  rain  every  year  during 
which  the  rain  amounts  each  day  to  0.1  inch  or  less,  while  at  Pieter¬ 
maritzburg  the  number  of  days  with  such  small  rainfall  each  year 
averages  57.  The  Cape  Town  type  of  rainfall,  with  modifications, 
especially  as  to  amount,  holds  in  a  general  way  in  western  South 
Africa,  while  the  Pietermaritzburg  type  is  characteristic,  also  with 
modifications,  for  central  and  eastern  South  Africa,  and  to  a  degree 
as  far  west  as  the  highlands  of  Southwest  Africa.  In  an  intermediate 
zone,  as  above  referred  to  and  elsewhere  mentioned  in  greater  detail, 
neither  the  one  or  the  other  type  prevails,  but  the  influence  of  both 
is  felt.  And  in  the  highlands,  especially  of  the  eastern  portion  of  the 
Union,  the  rainfall  is  fairly  uniformly  distributed  through  the  year. 


24 


FEATURES  OF  THE  VEGETATION  OF  THE 


RAINFALL  IN  THE  LITTLE  KARROO. 

Turning  now  from  the  consideration  of  certain  features  of  the  rain¬ 
fall  at  stations  on  or  near  the  coast,  we  will  examine  some  of  the  leading 
rainfall  characteristics  of  stations  in  the  interior,  and  first  of  all,  of 
certain  of  those  in  the  Little  Karroo.  Ladysmith,  Montagu,  Oudts- 
hoorn,  and  Uniondale  may  be  taken  to  represent  the  Little  Karroo,  so 
far  as  the  rainfall  of  that  region  is  concerned.  These  stations  are  sit¬ 
uated  in  a  fairly  east-west  line,  the  order  from  the  west  being  Montagu, 
Ladysmith,  Oudtshoorn,  and  Uniondale.  As  table  2  indicates,  the 
rainfall  at  Montagu  is  12.8  inches  annually,  at  Ladysmith  14.08  inches, 
at  Oudtshoorn  9.45  inches,  and  at  Uniondale  13.42  inches.  Situated 
as  they  are  in  the  zone  separating  the  two  well-marked  rainfall  regions 
on  the  east  and  on  the  west,  all  of  these  stations  show  to  a  degree 
the  influence  of  both  types  of  rainfall.  An  examination  of  table  2  will 
show  that  the  seasonal  percentages  of  precipitation  in  the  Little  Karroo 
exhibit  considerable  uniformity,  recalling  in  this  respect  the  stations 
nearer  the  coast.  At  Montagu  the  heaviest  rainfall  is  in  winter  and 
the  least  in  summer,  which  is  also  true  of  all  the  other  stations,  except 
only  Oudtshoorn,  at  which  the  winter  rainfall  is  the  least.  The 
average  length  of  the  drought  periods  of  winter  at  Montagu  and  Lady¬ 
smith  is  relatively  less  than  at  the  other  stations  farther  to  the  east, 
and  indicates  the  influence  of  the  western  type  of  rainfall.  As  to  the 
rainfall  extremes,  it  is  of  interest  to  note,  whatever  may  be  the  sig¬ 
nificance,  that  the  yearly  minima  for  both  stations  is  about  the  same, 
between  7  and  8  inches,  and  the  yearly  maxima  of  Montagu,  Lady¬ 
smith,  and  Oudtshoorn  lie  between  21  and  22.08  inches,  while  the 
maximum  at  Uniondale  is  about  28  inches.  The  seasonal  extremes  at 
Uniondale  appear  also,  on  the  whole,  to  be  the  most  marked,  the  maxi¬ 
mum  for  autumn  being  12.8  and  the  minimum  0.95,  and  the  maximum 
and  minimum  of  spring  being  10.7  and  0.67  inches.  Uniondale  has 
also  experienced  the  greatest  monthly  extremes  of  rainfall,  and  also 
one  of  the  heaviest  daily  precipitations  recorded  in  the  Union,  namely, 
7.67  inches.  The  length  of  the  average  period  each  year  having  0.15 
inch  per  month  rain',  or  no  rainfall,  for  Montagu  and  Oudtshoorn  is 
2.2  months,  and  for  Ladysmith  and  Uniondale  1.2  months.  The 
total  number  of  months  with  no  recorded  rainfall  at  Montagu  for  37 
years  is  59;  at  Ladysmith,  for  39  years,  17;  at  Oudtshoorn,  for  42 
years,  29;  and  at  Uniondale,  for  42  years,  16  months.  The  average  for 
these  stations  is,  respectively,  1.69,  0.43,  0.69,  and  0.38  months  each 
year  without  rain.  When  we  subtract  the  length  of  the  rainless  period 
from  the  length  of  the  period  of  drought,  as  here  defined,  to  get  the 
average  length  of  periods  having  less  than  0.15  inch  rain  each  year, 
it  appears  that  Montagu  has  the  least,  0.51  month,  and  Oudtshoorn 
the  greatest,  1.51  months.  This  accords  well  with  the  actual  number 
of  days  each  year  in  which  the  average  rainfall  is  0.10  inch,  or  less,  and 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


25 


suggests  that  the  average  rainfall,  per  storm,  is  larger  at  Montagu 
than  at  the  station  further  east.  The  following  are  the  average 
number  of  days  during  which  in  each  year  the  rain  amounts  to  less 
than  0.10  inch:  Montagu,  5.9;  Ladysmith,  19.4;  Oudtshoorn,  13.6; 
Uniondale,  18.9.  That  the  average  storm  may  be  somewhat  larger 
at  Montagu  and  smaller  at  Oudtshoorn  also  appears  from  the  data 
given  by  Marloth,1  from  which  it  may  be  found  that  the  average  daily 
storm  at  the  four  stations  in  the  order  above  given  is  as  follows: 
0.39,  0.29,  0.25,  and  0.28  inch. 

Without  entering  into  a  fuller  discussion  of  the  rainfall  of  the  Little 
Karroo,  it  will  be  apparent  that  as  a  whole  it  is  fairly  uniform,  or  at 
least  that  it  shows  less  marked  periodicity  than  stations  much  to  the 
west  or  to  the  east.  This  is  due  in  part  to  the  position  of  the  area  and 
also  in  part,  according  to  Marloth,2  to  the  influence  of  near  mountains, 
as  at  Ladysmith.  However,  the  station  is  clearly  under  the  influence 
of  the  western  type  of  rainfall,  as  would  appear  from  the  fact  that 
over  34  per  cent  of  the  rainfall  of  the  place  occurs  in  winter. 

RAINFALL  OF  THE  GREAT  OR  CENTRAL  KARROO. 

The  stations  which  have  been  selected  to  represent  the  rainfall 
conditions  of  the  Great  Karroo  are  Aberdeen  and  Graaf  Reinet  on  the 
extreme  east,  Laingsburg  and  Matjesfontein  on  the  extreme  west, 
and  Beaufort  West,  which  is  somewhat  to  the  east  of  the  central 
portion.  From  east  to  west,  as  mentioned  in  another  place,  the  Great 
Karroo  extends  for  a  distance  of  between  200  and  300  miles,  and  it 
lies  from  80  to  140  miles  from  the  south  coast,  being  nearer  on  the  west. 
The  situation  of  the  stations  is  such,  as  will  appear  directly,  that  those 
to  the  east  are  under  the  influence  of  the  eastern  type  of  rainfall,  and 
those  to  the  west  are  more  or  less  under  the  influence  of  the  western 
type;  that  is  to  say,  that  at  Aberdeen,  Graaf  Reinet,  and  Beaufort 
West  a  relatively  large  amount  of  rain  falls  during  the  warm  seasons 
(spring,  summer,  and  autumn),  while  at  Laingsburg  and  Matjes¬ 
fontein  there  is  a  relatively  large  percentage  of  rainfall  during  the 
winter  season,  although  in  the  west  the  distribution  of  the  rainfall 
throughout  the  year  is  much  more  uniform  than  at  the  stations  further 
east. 

ABERDEEN  AND  GRAAF  REINET. 

The  average  rainfall  at  Aberdeen  is  12.19  and  at  Graaf  Reinet  it  is 
13.87  inches.  At  Aberdeen  the  average  rainfall  of  summer  is  36.7  per 
cent  of  the  total,  and  at  Graaf  Reinet  it  is  29.6  per  cent.  The  winter 
rains  at  the  two  stations  are  8.9  and  8.6  per  cent  of  the  average, 
respectively.  The  rainfall  of  autumn  at  Aberdeen  is  30.4  per  cent 

1  A  guide  to  botanical  survey  work.  Bot.  Survey  So.  Africa,  Memoir  No.  4.  Issued  by  the 

advisory  committee  for  the  botanical  survey  of  South  Africa.  Pp.  40-42.  1922. 

2  Das  Kapland,  etc.  Wissensch.  Ergebnisse  deutsch.  Tiefsee-Expedition,  2d  Bd.,  3  Th., 

p.  257.  1908. 


26 


FEATURES  OF  THE  VEGETATION  OF  THE 


and  at  Graaf  Reinet  31.1  per  cent,  but  the  rain  in  spring  at  Aberdeen 
is  23.7  per  cent,  while  at  Graaf  Reinet  it  is  29.7  per  cent  of  the  annual 
average  rainfall.  The  differences  in  the  distribution  of  the  rain 
throughout  the  year  at  the  two  stations  can  presumably  be  accounted 
for  by  the  relation  to  the  mountains,  which,  in  the  case  of  Aberdeen, 
lie  to  the  north,  and  in  the  case  of  Graaf  Reinet  lie  to  the  east  and  to 
the  west  as  well. 

Turning  now  to  the  rainfall  extremes,  it  is  interesting  to  note  that 
the  maximal  rainfall  for  the  year,  for  the  seasons,  and  for  the  month, 
at  both  stations,  is  about  the  same;  the  same  is  true  also  for  the  minimal 
rainfall,  but  Graaf  Reinet  has  experienced  the  larger  daily  maximum. 
In  one  day  6.5  inches  of  rain  was  reported  to  have  fallen  at  Graaf 
Reinet,  while  the  largest  reported  rain  occurring  on  one  day  at  Aber¬ 
deen  was  3.9  inches.  There  is  a  considerable  apparent  difference 
between  the  two  stations  as  to  the  average  length  of  the  drought 
periods.  At  Aberdeen  the  average  yearly  dry  period  is  3  months;  at 
Graaf  Reinet  it  is  2.1  months.  As  for  the  seasonal  differences  in  the 
length  of  the  drought  periods,  it  will  be  seen  from  table  3  that  the  per¬ 
centage  in  winter,  for  the  two  stations,  is  almost  the  same,  namely, 
51.4  for  Aberdeen  and  51.7  for  Graaf  Reinet.  In  Spring,  at  Aberdeen, 
occurs  21.49  per  cent  of  the  drought  of  the  year,  and  at  Graaf  Reinet, 
for  the  same  season,  the  percentage  is  22.1.  In  summer,  however, 
the  drought  period  is  longer  at  Graaf  Reinet  than  at  Aberdeen,  12 
at  the  former  as  opposed  to  16.4  per  cent  at  the  latter  station,  but  in 
autumn  the  ratio  is  in  the  opposite  direction  and  is  considerably 
greater.  Thus,  at  Graaf  Reinet  the  autumnal  drought  period  is  8.3 
per  cent  of  the  whole  and  that  at  Aberdeen  is  14.9  per  cent.  These 
seasonal  differences  in  the  length  of  the  drought  period  at  Aberdeen 
and  Graaf  Reinet  correspond  very  well  with  the  differences  in  seasonal 
precipitation,  as  represented  by  percentages  of  the  total  rainfall  at  the 
stations,  as  is  shown  in  table  2,  and  may  have  for  their  leading  cause 
local  features,  such  as  variation  in  topography,  or  unlike  relation  to 
neighboring  highlands,  which,  as  was  pointed  out  above,  may  be 
influential  in  molding  certain  characteristics  of  the  rainfall,  as  at 
Ladysmith  and  other  places. 

At  Aberdeen  the  average  length  of  the  dry  spells  during  which 
no  rainfall  was  reported,  for  a  period  of  39  years,  is  1.4  and  at  Graaf 
Reinet  it  is  0.85  month.  By  subtracting  these  amounts  from  the 
drought  periods  as  given  in  table  3,  the  periods  during  which  the 
rainfall  averaged  less  than  0.15  inch  in  each  month  can  be  arrived  at. 
Thus,  in  the  case  of  Aberdeen  it  is  1.6  and  in  that  of  Graaf  Reinet  it  is 
1.25  month,  suggesting  that  at  the  former  station  there  may  be  more 
rain  of  this  character  than  at  the  latter. 

The  average  number  of  rainy  days  at  Aberdeen  is  41  and  at  Graaf 
Pceinet  52,  making  the  average  daily  amount  of  rain  during  rainy 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


27 


periods  at  the  two  places  0.29  and  0.25  inch,  respectively,  which  is 
possibly  to  be  attributed  to  the  greater  percentage  at  Aberdeen  of  sum¬ 
mer  precipitation.  The  frequency  of  long  spells  of  continuous  rain 
appears  to  be  characteristic.  Thus,  at  Aberdeen,  during  39  years, 
nine  such  spells  were  recorded,  with  an  aggregate  of  17.83  inches. 
These  long  stormy  spells  were  in  summer,  autumn,  and  spring.  At 
Graaf  Reinet,  in  40  years,  but  three  such  stormy  spells  were  recorded, 
with  an  aggregate  rainfall  of  5.05  inches,  occurring  in  summer  and  in 
spring. 

BEAUFORT  WEST. 

Beaufort  West  has  an  average  annual  rainfall  of  9.56  inches,  of 
which  31.8  per  cent  is  in  summer,  36.1  per  cent  in  autumn,  10  per  cent 
in  winter,  and  21.8  per  cent  in  spring.  The  actual  seasonal  amounts 
are:  2.9,  3.4,  0.9,  and  2  inches.  The  mean  annual  number  of  days 
on  which  rain  occurs  at  Beaufort  West,  according  to  Marloth,  cited 
above,  is  36,  which  gives  the  average  amount  of  rain  for  each  rainy 
day  as  0.36  inch.  But  the  amount  of  rain  falling  in  one  day  may 
be  very  considerable,  as,  for  example,  it  was  recorded  that  8.08  inches 
of  rain  occurred  in  24  hours  at  Beaufort  West,  which  is  the  heaviest 
day’s  rain  of  the  stations  considered.  This,  it  will  be  observed,  is 
nearly  the  average  for  a  year  at  this  station.  As  much  as  9.7  inches 
has  been  recorded  in  one  month.  An  examination  of  table  4,  giving 
rainfall  extremes,  will  show  that  the  maxima  and  the  minima  for  the 
four  seasons  are  all  very  well  marked,  the  greatest  possibly  occurring 
in  winter.  Over  a  period  of  43  years,  two  rainy  spells  of  5  days  each 
were  reported,  which  aggregated  2.98  inches.  At  Beaufort  West  there 
is  an  average  of  16  rainy  days  during  which  less  than  0.1  inch  of  rain 
fell.  At  Beaufort  West,  also,  there  are  3.1  months  every  year  which 
constitute  the  drought  period.  During  1.3  months,  taking  the  average 
of  43  years,  no  rainfall  occurs  at  Beaufort  West.  The  seasonal  dis¬ 
tribution  of  the  drought,  which  is  elsewhere  defined,  is:  in  summer 
19.8,  in  autumn  13.9,  in  winter  40,  and  in  spring  25.7  per  cent.  The 
actual  seasonal  precipitation  at  Beaufort  West  is  2.9  inches  in  summer, 
3.4  inches  in  autumn,  0.9  inch  in  winter,  and  2  inches  in  spring. 

LAINGSBURG. 

At  Laingsburg  the  average  rainfall  is  4.47  inches,  which  is  dis¬ 
tributed  through  the  year  as  follows:  19.9  per  cent  in  summer,  35.8 
per  cent  in  autumn,  27.5  per  cent  in  winter,  and  16.5  per  cent  in  spring. 
The  actual  rainfall  for  the  four  seasons  in  order  as  above  given  is: 
0.8  inch,  1.6  inches,  1.2  inches,  and  0.7  inch,  from  which  it  will  appear 
that  the  most  rain  is  in  autumn  and  winter.  This,  however,  is  in¬ 
considerable,  and  Laingsburg  is  one  of  the  most  arid  stations  of  the 
Great  Karroo. 


28 


FEATURES  OF  THE  VEGETATION  OF  THE 


The  average  yearly  drought  period  at  Laingsburg  is  5.2  months, 
of  which  34  per  cent  is  in  summer,  17  per  cent  each  in  autumn  and 
winter,  and  31.9  per  cent  in  spring.  As  for  the  length  of  the  average 
drought  period,  that  at  Laingsburg  is  exceeded  by  few  stations  in  the 
Union,  of  which  may  be  mentioned  O’okiep  and  Upington.  At  Laings¬ 
burg,  and  for  a  period  of  observations  covering  18  years,  in  61  months 
no  rainfall  whatever  was  recorded,  which  is  an  average  of  3.3  months 
yearly.  Thus  the  average  length  of  the  rainless  season  at  Laingsburg 
is  approximately  2.4  times  that  of  Beaufort  West. 

The  number  of  days  yearly  during  which  0.1  inch  of  rain  falls  at 
Laingsburg  is  9.3,  as  compared  with  16  at  Beaufort  West.  The 
longest  rainy  spells  at  Laingsburg  are  of  four  days,  of  which  two 
aggregating  1.62  inches  precipitation  have  been  reported. 

As  might  be  expected,  the  rainfall  extremes  at  Laingsburg  are  very 
well  marked.  The  maximum  precipitation  recorded  for  a  year  is 
7.1  inches  and  the  minimum  precipitation  is  1.06  inches.  The  maxi¬ 
mum  for  summer  is  2.8  inches,  for  winter  2.19  inches,  and  for  spring 
2  inches,  with  no  precipitation,  in  each  instance,  as  the  opposite 
extreme.  The  maximum  recorded  for  autumn  is  4.36  inches,  with 
0.29  inch  as  the  minimum.  The  maximum  monthly  rainfall  reported 
for  Laingsburg  is  3.18  inches  and  the  heaviest  rain  for  24  hours  at 
Laingsburg  is  1.75  inches. 

MATJESFONTEIN. 

The  average  annual  rainfall  at  Matjesfontein  is  6.8  inches,  which 
is  distributed  through  the  year  as  follows:  17.8  per  cent  in  summer, 
33.3  per  cent  in  autumn,  25  per  cent  in  winter,  and  23.6  per  cent  in 
spring.  The  actual  amounts  of  rainfall  are  1.1,  2.1,  1.6,  and  1.5  inches 
for  the  four  seasons,  respectively.  From  this  it  will  be  seen  that 
the  rainfall  at  Matjesfontein  resembles  that  of  Laingsburg  in  dis¬ 
tribution  through  the  year,  but  is  somewhat  larger  in  amount.  As  at 
Laingsburg,  the  extremes  in  the  reported  amount  of  rainfall  are  very 
considerable;  thus,  the  maximum  yearly  rainfall  at  Matjesfontein  is 
10.38  inches  and  the  minimum  2.16  inches.  The  maximum  rainfall  of 
summer  is  4.9,  for  autumn  6.1,  for  winter  5.1,  and  for  spring  4.7  inches, 
with  no  rain  for  the  opposite  extreme.  The  greatest  rainfall  for  a 
month  is  3.3  inches1  and  in  24  hours  2.06  inches.  At  Matjesfontein, 
in  33  years,  there  has  been  one  rainy  spell  of  5  days’  duration,  two  of 
6  days’  duration,  and  one  of  7  days,  which  is  strongly  opposed  to  the 
condition  at  Laingsburg,  with  no  rainy  period  more  than  4  days  in 
length. 

The  length  of  the  average  yearly  period  of  drought  is  3.9  months, 
of  which  35.3  per  cent  is  in  summer,  20  per  cent  is  in  autumn,  15.3 
per  cent  is  in  winter,  and  29.2  per  cent  is  in  spring.  At  Matjesfontein 


1  June  1921,  4.30  inches  rain  at  Matjesfontein. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


29 


also  there  are  on  the  average  three  months  in  which  no  rain  occurs, 
from  which  it  will  be  seen  that  the  amount  of  rain  falling  in  amounts 
of  less  than  0.15  inch  in  a  month  are  relatively  small,  although  there 
are  on  the  average  18.8  days  each  year  with  less  than  0.1  inch  rainfall. 
As  has  already  been  pointed  out,  the  average  annual  rainfall  at  Matjes- 
fontein  is  6.8  inches  and  that  of  Laingsburg  is  4.4  inches.  The 
average  annual  rainfall  at  the  latter  station  is  therefore  2.4  inches 
less  than  at  the  former  station,  although  the  two  are  but  18  miles 
apart. 

Table  5. — Drought  'periods  ( periods  with  less  than  0.15  inch  rainfall  in  a  month). 


Maximum 
length  of 
continuous 
drought,  in 
months. 

Yearly  average 
number  separate 
continuous 
drought  periods, 

1  month  or 
more  each. 

Total  number 
drought  periods, 

1  month  or 
longer.  (Length 
of  record 
years,  in 
parenthesis.) 

Cape  Town . 

3 

0.59 

35 

(59) 

Grahamstown . 

2 

0.18 

8 

(43) 

Pietermaritzburg . 

3 

0.92 

24 

(26) 

Ladysmith . 

3 

0.97 

40 

(39) 

Montagu . 

4 

1.50 

56 

(37) 

Oudtshoorn . 

4 

1.50 

64 

(42) 

Uniondale . 

2 

0.95 

44 

(42) 

Aberdeen . 

4 

1.80 

71 

(39) 

Graaf  Reinet . 

4 

1.50 

62 

(40) 

Beaufort  West . 

5 

2.10 

94 

(43) 

Laingsburg . 

5 

2.80 

51 

(18) 

Matjesfontein . 

6 

2.30 

78 

(23) 

O’okiep . 

8 

2.30 

91 

(38) 

Upington . 

7 

2.40 

68 

(28) 

Pretoria . 

4 

1.40 

26 

(18) 

Pietersburg . 

6 

1.30 

23 

(17) 

Messina . 

7 

1.50 

17 

(11) 

The  rainfall  between  these  t%o  stations  is  apparently  graduated,  de¬ 
creasing  in  amount  as  one  goes  east  toward  Laingsburg.  Going  west¬ 
ward  from  Matjesfontein,  the  opposite  condition  obtains,  the  rainfall 
increasing  in  a  marked  manner  in  short  distances.  For  example, 
for  two  years  ending  in  June  1921,  the  total  rainfall  at  Matjesfontein 
was  20.49  inches,  while  at  a  station  2  miles  to  the  west  it  was  25.64 
inches,  at  a  second  station  4  miles  west  it  was  30.39  inches,  at  a  third 
station  5  miles  west  it  was  31.4  inches,  and  at  a  fourth  station  7  miles 
west  it  was  33.78  inches.  That  is,  in  a  horizontal  distance  of  approxi¬ 
mately  7  miles  a  total  difference  of  rainfall  in  two  years  was  found  to 
be  about  13  inches.  Without  entering  into  an  account  of  the 
topography  of  the  region  immediately  about  Matjesfontein,  which  is 
touched  on  on  another  page,  it  can  be  said  that  while  the  altitude  at 
Matjesfontein  is  2,955  feet,  that  of  the  station  7  miles  to  the  west  is 
3,585  feet,  and  that  the  hills  both  to  the  north  and  to  the  south  are 


30 


FEATURES  OF  THE  VEGETATION  OF  THE 


nearer  the  western  station.  These  differences  in  rainfall  have  a  very 
striking  effect  on  the  kind  and  the  amount  of  the  vegetation,  as  will 
appear  elsewhere. 

DROUGHT  PERIODS. 

The  length  of  the  continuous  drought  periods,  that  is,  the  period 
(number  of  months)  in  a  year  either  with  no  rainfall  or  with  less  than 
0.15  inch  of  rain,  is  extremely  variable  as  between  the  different  stations, 
more  especially  in  the  arid  regions  of  southern  Africa.  This  can  be 
illustrated  by  reference  to  the  records  for  a  few  representative  sta¬ 
tions. 

The  least  number  of  months  with  no  rain,  or  with  less  than  0.15 
inch  of  rain  is  at  Grahamstown,  where  in  43  years  only  eight  months 
are  reported  as  being  months  of  drought  in  the  sense  here  understood. 
A  drought  period  occurs  about  once  in  five  years  at  Grahamstown. 
The  maximum  recorded  length  of  a  drought  period  at  Grahamstown  is 
about  two  months. 

At  Cape  Town,  the  average  yearly  periods  of  drought  in  months 
are  four  times  the  length  of  those  at  Grahamstown.  At  the  former  the 
drought  period  is  0.23  and  at  the  latter  it  is  0.93  month. 

The  maximum  length  of  a  drought  period  at  Cape  Town  is  about 
three  months  and  at  Pietermaritzburg  it  is  about  the  same.  These 
are  the  lowest  maxima  of  any  stations  under  discussion. 

At  Ladysmith,  Montagu,  Oudtshoorn,  and  Uniondale,  in  the  Little 
Karroo,  the  maximum  length  of  continuous  drought  periods  are  3, 
4,  4,  and  2  months  for  these  stations  respectively.  At  the  two  former 
stations  there  are  on  the  average  about  three  drought  periods  in  two 
years,  and  at  the  latter  stations  there  is  about  one  such  period  every 
year. 

Of  the  stations  in  the  Great  Karroo,  the  maximum  length  of  the 
drought  periods  is  4  months  each  at  Aberdeen  and  Graaf  Reinet,  5 
months  each  at  Beaufort  West  and  Laingsburg,  and  at  Matjesfontein 
it  is  6  months.  Of  these  stations  the  yearly  average  number  of  separate 
drought  periods  is  least  at  Graaf  Reinet,  with  the  number  at  Aberdeen, 
Beaufort  West,  Matjesfontein,  and  Laingsburg  successively  larger. 
Thus,  the  order  of  the  number  of  separate  periods  of  drought  for  these 
stations  is  the  converse  of  the  annual  rainfall  occurring  at  them. 

At  O’okiep,  observations  covering  38  years,  91  separate  drought 
periods  have  been  reported,  or  an  average  of  2.3  each  year.  Of  these 
there  are  one  of  8,  one  of  6,  four  of  5,  and  two  of  4  months  duration. 
They  are  mainly  in  the  warm  seasons. 

In  28  years,  68  drought  periods  were  reported  at  Upington.  There 
were  two  of  7  months  duration,  one  of  6,  five  of  5,  and  three  of  4 
months.  The  greatest  number  of  drought  periods  at  Upington  is 
in  the  cool  seasons. 

-  r 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


31 


At  Pretoria,  in  18  years,  there  were  26  periods  of  drought,  with  four 
months  as  the  maximum.  At  Pietersburg,  during  17  years,  the 
number  of  drought  periods  was  23  and  the  longest  was  6  months.  At 
the  latter  station  there  were  four  periods  of  4  months,  three  of  5 
months,  and  one  of  6  months  duration. 

At  Messina,  in  11  years,  there  were  17  drought  periods  with  7 
months  as  the  longest.  Of  these  there  were  two  of  4  months,  two  of 
5  months,  two  of  6  months,  and  one  of  7  months  duration.  Messina, 
thus,  with  14.5  inches  rainfall,  has  somewhat  larger  "drought  expec¬ 
tancy  than  Pietersburg,  or  Pretoria,  at  which  the  rainfall  is  21,  18, 
and  29.2  inches,  respectively.  But  Pretoria,  which  thus  has  a  larger 
rainfall  than  Pietersburg,  has  also  a  relatively  larger  number  of  drought 
periods,  reversing  the  rule  referred  to  in  a  preceding  paragraph. 


32 


FEATURES  OF  THE  VEGETATION  OF  THE 


RAINFALL  IN  THE  PROTECTORATE  OF  SOUTHWEST 
AFRICA  AND  IN  THE  NORTHWESTERN 
PART  OF  THE  UNION. 

A  vast  region  stretches  northward  from  the  Great  Karroo  to  and 
beyond  the  Tropic  of  Capricorn,  reaching  to  the  Atlantic  Ocean,  in 
which  the.  rainfall  is  10  inches,  or  less,  annually.  The  amount  of 
precipitation  decreases  from  the  south  to  the  north,  and  from  the  west 
to  the  east.  The  exceptions  to  these  generalizations  occur  in  the  Pro¬ 
tectorate  of  Southwest  Africa,  where  in  the  highlands  there  is  higher 
rainfall,  and  in  the  extreme  south  of  Cape  Colony,  in  which,  in  part 
at  least,  for  a  similar  reason,  irregularities  exist.  In  the  Protectorate 
are  to  be  found  the  most  arid  portions  of  South  Africa,  if  not  of  the 
Southern  Hemisphere,  lying  between  the  highlands  and  the  ocean. 
It  is  the  Namib  Desert  and  its  southern  extension.  Thus,  at  Lue- 
deritz  Bay,  the  annual  rainfall  is  given  by  Knox  as  0.79  inch  and 
Swakopmund  0.83  inch.  In  the  highlands,  however,  the  rainfall  in 
places  exceeds  20  inches. 

The  stations  of  which  the  rainfall  will  be  especially  spoken  of  in 
this  review  are  O’okiep,  in  Namaqualand,  south  of  the  Orange  River 
and  distant  in  a  straight  line  about  50  miles  from  the  Atlantic  Ocean, 
and  Upington,  on  the  Orange  River,  distant  about  250  miles  from  the 
ocean,  both  of  which  are  in  the  Union  of  South  Africa,  and  the  fol¬ 
lowing  stations  in  Great  Namaqualand,  or  the  Protectorate:  Warmbad, 
about  120  miles  from  the  ocean  and  about  50  miles  north  of  the  Orange 
River;  Keetmanshoop,  about  150  miles  from  the  ocean  and  50  miles 
north  of  Warmbad;  Bethany  and  Gibeon,  lying  between  Keetman¬ 
shoop  and  Windhoek;  Windhoek,  160  miles,  and  Gobabis,  about  250 
miles,  in  a  straight  line  from  the  Atlantic  Ocean;  Karibib,  Swakop¬ 
mund,  and  Luederitz  Bay.  Karibib  is  between  Windhoek  and  Swa¬ 
kopmund,  which  is  on  the  ocean.  Luederitz  Bay,  about  250  miles 
south  of  Swakopmund,  lies  nearly  west  of  Bethany  and  northwest  of 
Keetmanshoop  and  about  500  miles  north  of  Cape  Town. 

O’OKIEP. 

The  altitude  above  sea-level  of  O’okiep  is  3,025  feet,  of  Upington  is 
2,641  feet,  of  Warmbad  2,361  feet,  of  Keetmanshoop  3,286  feet,  of 
Gibeon  3,707  feet,  of  Windhoek  5,428  feet,  of  Gobabis  4,649  feet,  of 
Karibib  3,842  feet,  and  that  of  Bethany  not  far  from  3,500  feet. 

The  rainfall  at  O’okiep  is  6.4  inches,  the  average  of  38  years’  records, 
distributed  through  the  four  seasons  as  follows:  in  summer,  0.6  inch; 
in  autumn,  2  inches;  in  winter  2.6  inches;  and  in  spring  1.2  inches. 
There  thus  occurs  9.3  per  cent  in  summer,  31.2  per  cent  in  autumn, 
40.6  per  cent  in  winter,  and  18.7  per  cent  in  spring.  The  extremes 
in  rainfall  at  O’okiep  are  very  well  marked.  The  maximum  yearly 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


33 


rainfall  is  11.7  and  the  minimum  2.46  inches.  The  rainfall  maxima 
of  summer,  autumn,  winter,  and  spring  are  2.5,  6.04,  6.17,  and  3.3 
inches,  and  the  seasonal  rainfall  minima  are  nothing  in  summer,  0.6 
inch  in  autumn,  0.7  in  winter,  and  0.1  inch  in  spring.  The  maximal 
fall  for  one  month  was  3.49  inches,  and  the  heaviest  rainfall  experi¬ 
enced  in  24  hours  was  2.05  inches.  The  drought  period  is  5.9  months 
yearly,  of  which  no  rainfall  occurs  in  1.6  months;  42  per  cent  of  the 
drought  period  occurs  in  summer,  17.4  per  cent  in  autumn,  9.6  in 
winter,  and  30  per  cent  in  spring.  Thus  O’okiep  is  within  the  region 
of  winter  rains.  There  are  at  O’okiep  about  41  rainy  days  each  year, 
of  which  the  daily  rainfall  on  23  days  is  0.1  inch  and  less,  and  on  8  days 
the  rainfall  is  from  0.11  to  0.20  inch.  This  makes  a  very  small  average 
daily,  about  0.1  inch,  with  a  relatively  large  number  of  rainy  days. 

WARMBAD. 

The  average  rainfall  at  Warmbad  for  11  years  is  3.62  inches.  There 
are  34  rainy  days  in  the  year,  making  the  average  daily  rainfall  0.16 
inch.  The  maximum  and  the  minimum  yearly  precipitation  at 
Warmbad  are  6.04  and  0.69  inches.  Summarized  monthly  rainfall 
data,  1913-1920,  show  that  there  are  two  rainy  periods  and  two  dry 
periods  in  the  year.  The  primary  period  of  little  rain  is  in  early  spring, 
September,  when  the  average  precipitation  is  0.03  inch.  The  monthly 
rains  then  increase  gradually  in  amount  up  to  and  including  December, 
when  0.91  inch  occurs.  They  are  somewhat  less  in  January,  0.35  inch, 
and  then  increase  to  the  maximum  in  March,  early  autumn,  when  the 
average  is  2.03.  The  monthly  amount  of  precipitation  decreases 
rapidly,  so  that  in  April  the  average  is  0.47  inch  and  in  May  and 
June  it  is  0.08  inch.  For  the  period  1913-1920,  the  yearly  precipita¬ 
tion  was  5.83  inches,  distributed  through  the  seasons  as  follows: 
2.03  inches  in  summer,  2.58  inches  in  autumn,  0.56  inch  in  winter,  and 
0.66  inch  in  spring.  Thus  there  is  34.8  per  cent  in  summer,  44.3  per 
cent  in  autumn,  9.6  per  cent  in  winter,  and  11.1  per  cent  in  spring. 
The  heaviest  rain  of  summer  is  in  December,  and  the  heaviest  rainfall 
of  the  cool  season  in  March. 

KEETMANSHOOP. 

At  Keetmanshoop  the  average  rainfall  for  the  11  years,  1908-1920, 
was  4.84  inches,  the  maximum  being  8.6  and  the  minimum  1.2  inches. 
The  average  rainfall,  however,  for  the  years  1913  to  1920  was  6.43 
inches.  This  was  distributed  through  the  seasons  as  follows :  3  inches 
in  summer,  2.2  inches  in  autumn,  0.1  inch  in  winter,  and  1.09  inches 
in  spring.  Thus,  46.6  per  cent  of  the  annual  precipitation  at  Keet¬ 
manshoop  occurred  in  summer,  34.9  per  cent  in  autumn,  1.5  per  cent  in 
winter,  and  16.9  per  cent  in  spring. 


34 


FEATURES  OF  THE  VEGETATION  OF  THE 


BETHANY. 

The  average  rainfall  at  Bethany,  1908-1920,  was  3.9  inches.  The 
maximum  for  that  period  was  9.6  and  the  minimum  1.18  inches.  For 
the  period  1913-1920  the  average  rainfall  at  Bethany  was  5.43  inches. 
Of  this,  2.7  inches  occurred  in  summer,  1.8  inches  in  autumn,  0.1  inch 
in  winter,  and  0.77  inch  in  spring.  Thus,  50  per  cent  of  the  rainfall 
is  in  summer,  33.4  per  cent  in  autumn,  1.8  per  cent  in  winter,  and  14.3 
per  cent  in  spring. 

GIBEON. 

At  Gibeon  the  average  rainfall,  1908-1920,  was  6.36  inches.  The 
maximum  was  14.35  and  the  minimum  2.21  inches.  Of  the  period 
1913-1920,  the  average  annual  rainfall  was  7.7  inches,  of  which  4.11 
inches  was  in  summer,  2.69  inches  in  autumn,  0.11  in  winter,  and  0.79 
in  spring;  from  which  it  will  be  seen  that  53.3  per  cent  of  the  yearly 
rainfall  occurs  in  summer,  34.8  per  cent  in  autumn,  1.4  per  cent  in 
winter,  and  10.2  per  cent  in  spring.  There  is  thus  a  relatively  small 
amount  of  rain  in  winter,  as  in  the  last  two  stations  especially,  and  a 
relatively  large  proportion  in  the  warmer  seasons. 

WINDHOEK. 

At  Windhoek  the  average  rainfall  for  the  period  1908-1920  was 
13.37  inches,  the  maximum  22.45,  and  the  minimum  5.7  inches.  For 
the  period  1913-1920  the  average  annual  rainfall  was  14.67  inches. 
It  was  distributed  as  follows:  8.89  inches  in  summer,  4.2  inches  in 
autumn,  0.02  inch  in  winter,  and  1.56  inches  in  spring.  From  this  it 
will  be  seen  that  60.59  per  cent  occurs  in  winter,  28.6  per  cent  in 
autumn,  0.13  per  cent  in  winter,  and  10.6  per  cent  in  spring. 

GOBABIS. 

At  Gobabis,  for  the  period  1908-1920,  the  average  rainfall  was 
15.53  inches.  The  maximum  rainfall  for  the  period  was  26.6.  The 
average  rainfall  at  Gobabis  for  the  period  1913-1920  was  15.48  inches, 
which  was  distributed  through  the  seasons  as  follows:  9.24  inches  in 
summer,  4.51  inches  in  autumn,  no  rainfall  in  winter,  and  1.72  inches 
in  spring.  From  this  it  will  be  seen  that  in  summer  there  occurs  59.6 
per  cent,  in  autumn  29.1  per  cent,  and  in  spring  11.1  per  cent. 

KARIBIB. 

The  average  rainfall  at  Karibib  for  1908-1920  was  7.38  inches. 
The  rainfall  extremes  for  this  period  were  12.49  and  2.62  inches. 
For  the  period  1913-1920,  the  average  annual  rainfall  at  Karibib 
was  8.71  inches,  of  which  5.09  inches  occurred  in  summer,  2.75  inches 
in  autumn,  there  was  no  rainfall  in  winter,  and  in  spring  0.87  inch. 
Therefore  58.4  per  cent  of  the  rainfall  at  Karabib  occurs  in  summer, 
31.5  per  cent  in  autumn,  and  10  per  cent  in  spring. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


35 


SWAKOPMUND. 

At  Swakopmund  for  the  period  1908-1920  the  average  annual 
rainfall  was  0.69  inch.  The  maximum  rainfall  for  this  period  was  2.41 
inches  and  the  minimum  was  0.01  inch.  For  the  period  1913-1920 
the  average  annual  rainfall  was  1.02  inch,  of  which  0.72  inch  was 
in  summer,  0.18  inch  in  autumn,  no  rain  in  winter,  and  0.12  inch  in 
spring.  From  this  it  will  appear  that  while  the  rainfall  at  Swakop¬ 
mund  is  very  small  indeed,  it  mainly  occurs  in  the  warm  season,  that 
is,  at  the  time  of  the  heaviest  rains  in  the  highlands  to  the  east.  Feb¬ 
ruary  is  the  most  rainy  month. 

LUEDERITZ  BAY. 

The  average  rainfall  for  the  period  1908-1920  at  Leuderitz  Bay 
was  1.97  inches.  The  maximum  rainfall  for  a  year  was  4.31  inches 
and  the  minimum  0.67  inch.  Of  the  period  1913-1920  the  average 
rainfall  was  2.95  inches,  of  which  0.93  inch  occurred  in  summer,  0.54 
inch  in  autumn,  1.27  inches  in  winter,  and  0.21  inch  in  spring.  From 
this  it  will  be  seen  that  the  rains  of  summer  are  31.5  per  cent,  of  autumn 
18.1  per  cent,  of  winter  43  per  cent,  and  of  spring  7.1  per  cent. 

The  average  number  of  rainy  days  for  the  period  1908-1920,  11 
years,  at  the  Protectorate  stations  given  above,  varies  from  29  to  70. 
Warmbad  and  Luederitz  Bay  have  the  least  and  Gobabis  the  largest 
number  of  rainy  days  per  year.  At  Keetmanshoop  the  number  was 
36  and  at  Bethany  it  was  30,  at  Gibeon  it  was  46,  at  Windhoek  66, 
at  Karibib  43,  and  at  Swakopmund  50.  The  average  amount  of 
rainfall  per  rainy  day  varied  from  Swakopmund  with  0.01  and  Lueder¬ 
itz  Bay  with  0.06  inch,  to  Gobabis  with  0.22  inch.  At  Grootfontein, 
however,  which  is  about  200  miles  north  of  Windhoek  and  about  350 
miles  from  the  coast,  the  rainfall  is  20.34  inches,  with  77  rainy  days. 
Period  1908-1920,  the  daily  average  precipitation  on  rainy  days,  there¬ 
fore,  is  0.26  inch.  But  the  heaviest  average  rainfall  of  the  Protec¬ 
torate  is  Omapunda  with  607  mm.,  or  23.9  inches. 

DROUGHT  PERIODS  IN  THE  PROTECTORATE. 

The  drought  periods,  that  is,  the  number  of  months  without  precipi¬ 
tation  or  with  less  than  0.15  inch,  constitutes  a  very  prominent  char¬ 
acteristic  of  the  climatic  conditions  of  the  Protectorate.  The  follow¬ 
ing  remarks  on  the  subject  are  taken  from  records  covering  the  period 
1913-1920.  In  this  period  Grootfontein  and  Gobabis  had  each  28 
months  of  drought,  which  was  the  least  drought  aggregate,  and  at 
Swakopmund  the  number  of  months  with  drought  was  47,  the  largest 
drought  aggregate.  Owing  to  the  occurrence  of  rains  for  the  most  part 
in  the  warm  seasons,  the  drought  periods  are  mainly  in  the  colder 
seasons,  especially  in  winter.  At  Windhoek,  Gobabis,  Karibib,  and 
Grootfontein  there  is  no  drought  in  summer,  but  from  50  to  64  per  cent 


36 


FEATURES  OF  THE  VEGETATION  OF  THE 


of  the  drought  is  in  winter.  At  the  other  stations  mentioned  above 
the  drought  in  winter  is  from  23  to  48.4  per  cent,  increasing  from  the 
south  to  the  north.  Thus  at  Luederitz  Bay  the  percentage  of  winter 
drought  is  23,  at  Warmbad  it  is  26.4,  at  Keetmanshoop  it  is  46.8, 
at  Bethany  it  is  45.4,  and  Gibeon  48.6,  with  52.1  per  cent  and  more  at 
Karibib  and  other  stations  as  mentioned  above.  At  Swakopmund  the 
percentage  of  winter  drought  is  31.8.  At  both  Swakopmund  and 
Luederitz  Bay  about  15  per  cent  of  the  drought  periods  occur  in 
summer.  The  drought  in  a  single  continuous  period  is  naturally  often 
considerable,  varying  between  5  and  9,  and  possibly  more.  At 
Swakopmund,  during  the  period  1913-1920,  in  three  years  in  which 
the  complete  records  are  at  hand,  there  were  9  months  of  continuous 
drought,  that  is,  with  less  than  0.15  inch  rainfall  per  month,  with  an 
annual  average  drought  aggregate  of  9.5  months.  It  will  be  rightly 
concluded,  from  what  has  been  said  above,  that  the  seasons  of  drought 
include  winter  arid  do  not  include  summer,  but  may  include  months 
from  autumn  or  spring. 

EFFECTIVE  PRECIPITATION. 

As  stated  elsewhere,  the  special  features  characteristic  of  rainfall 
in  arid  regions  are  not  only  the  small  amount  and  its  variations  from 
season  to  seasoni,  but  also  the  possibly  great  differences  in  the  amount 
of  separate  storms.  The  circumstance  last  referred  to  is  of  some 
ecological  importance. 

In  arid  regions  two  classes  of  precipitation  are  of  little  direct  value 
to  vegetation,  namely,  heavy  rains  of  short  duration,  on  the  one  hand, 
and  inconsequential  showers  on  the  other.  In  the  former  the  rains 
fail  to  moisten  the  soil  deeply  because  of  rapid  run-off,  and  in  the 
latter  case  because  they  are  of  too  small  amount.  The  possible  indi¬ 
rect  effect  of  such  types  of  rain,  especially  of  that  last  named,  is  not 
here  considered,  inasmuch  as  in  the  long  run  the  vegetation  of  an  arid 
region  is  dependent  on  the  moisture  of  the  soil  for  its  water-supply. 

For  the  purpose  of  defining  the  amount  of  rainfall  directly  beneficial 
to  vegetation  in  an  arid  region,  and  in  this  case  cloudbursts  and  the 
like  are  not  taken  into  consideration,  the  effective  rainfall  has  been 
arbitrarily  taken  as  consisting  of  0.15  inch  or  more,  which  occurs  in  a 
single  stormy  period.1  Amounts  under  0.15  inch  are  considered 
to  be  non-effective.  The  effective  rainfall  is  derived  by  subtracting 
the  total  non-effective  rainfall  from  the  precipitation. 

In  table  6  are  given  the  mean  annual  precipitation,  1913-1922  (P), 
and  the  mean  annual  effective  (E)  and  non-effective  (N-E)  pre¬ 
cipitation,  together  with  ratios  based  on  the  same.  In  the  last  column 
are  given  maximal  ratios  of  effective  and  non-effective  ratios  for  the 
period. 

1  Plant  habits  and  plant  habitats  in  the  arid  portions  of  South  Australia.  W.  A.  Cannon. 
Carnegie  Inst.  Wash.  Pub.  No.  308,  p.  48,  1921. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


37 


The  data  appear  to  indicate  that  there  is  a  relation  between  the 
seasonal  distribution  of  the  rainfall  and  the  relative  amount  that  is 
effective.  Thus,  in  regions  of  summer  rains  the  effective  precipitation 
is  relatively  large,  but,  on  the  other  hand,  in  regions  of  winter  rains 
it  may  be  relatively  small. 


Table  6. — Mean  annual  'precipitation  (P);  mean  effective  (E),  and  mean  non-effective  (N-E) 
precipitation,  with  E/P  and  N-E/P,  and  maximum  N-E/E  ratios,  1913-1922.a 


P.,  in. 

E.,  in. 

N-E.,  in. 

Per  cent 

mean 

E/P. 

Per  cent 
mean 
N-E/P. 

Per  cent 
maximum 
N-E/E. 

Cape  Town . 

26.39 

23.47 

2.92 

89 

11 

16 

Grahamstown . 

29.87 

26.52 

3.35 

89 

11 

18 

Pietermaritzburg .... 

44.51 

40.20 

4.24 

90 

10 

15 

Montagu . 

13.62 

12.35 

1.27 

90 

10 

19 

Ladysmith,  C.  P . 

15.83 

14.69 

1.14 

92 

8 

20 

Oudtshoorn . 

11.83 

10.44 

1.39 

88 

12 

22 

Uniondale . 

14.45 

12.46 

1.99 

86 

14 

27 

Matjesfontein . 

9.08 

8.0 

1.08 

88 

12 

27 

Laingsburg . 

5.36 

4.75 

0.61 

89 

11 

27 

Beaufort  West . 

12.50 

10.85 

1.65 

87 

13 

29 

Graaf  Reinet . 

15.41 

13.85 

1.56 

90 

10 

21 

Aberdeen . 

13.35 

12.14 

1.21 

91 

9 

22 

Upington . 

7.43 

6.56 

0.86 

88 

12 

17 

O’okiep . 

8.95 

7.49 

1.46 

84 

16 

25 

Pretoria . 

31.84 

29.43 

2.41 

92 

8 

9 

Pietersburg . 

23.09 

21.82 

1.67 

94 

6 

13 

a  Based  on  data  supplied  by  the  Meteorological  Office,  Pretoria. 


It  is  likely  that  in  regions  either  with  rain  in  summer  or  in  winter, 
the  region  having  the  larger  amount  has  also  relatively  more  effective 
rain  than  the  one  having  a  small  amount.  For  example,  the  pre¬ 
cipitation  at  Beaufort  West  is  9.46  inches  and  at  Graaf  Reinet  it  is 
13.98  inches.  In  the  former  place  65  per  cent  occurs  in  summer  and 
in  the  latter  66  per  cent.  At  Beaufort  West  the  precipitation  is  87 
per  cent  and  at  Graaf  Reinet  it  is  90  per  cent  effective.  It  is  as  if 
the  former  station  had  a  rainfall  of  8.2  inches  and  the  latter  a  rainfall 
of  12.5  inches,  both  being  equally  effective.  Beaufort  West,  conse¬ 
quently,  is  somewhat  more  arid,  in  reference  to  the  rainfall  only,  as 
compared  with  Graaf  Reinet,  than  would  be  indicated  by  the  precipi¬ 
tation  at  the  two  stations  only. 

The  mean  relative  effectiveness  of  the  rainfall  is  surprisingly  high, 
taken  as  a  whole.  In  the  Karroos,  for  example,  it  ranges  between  86 
and  92  per  cent.  Even  at  Laingsburg,  where  the  annual  precipitation 
is  extremely  low,  it  is  about  89  per  cent  effective.  About  52  per  cent 
of  the  rainfall  at  this  place  is  in  summer. 

The  extreme  differences  between  the  effective  and  the  non-effective 
rainfall  for  a  year  at  one  place  indicate  that  occasionally  there  may 
be  years  in  which,  owing  to  the  large  total  amount  of  the  latter,  the 
moisture  available  to  the  use  of  plants  is  markedly  below  the  supposed 


38 


FEATURES  OF  THE  VEGETATION  OF  THE 


amount  as  indicated  by  the  precipitation  taken  altogether.  This 
feature  is  brought  out  in  the  last  column  of  table  6.  It  will  be  seen  that 
from  27  to  29  per  cent  of  the  rainfall  in  the  Karroos  for  a  year  may  be 
non-effective.  It  is  so  great  moisture  deficiency  in  unusually  dry 
years  or  seasons,  occasioned  in  part  by  relatively  large  proportion 
of  non-effective  precipitation,  that  may  turn  the  scales  in  determin¬ 
ing  the  suitability  of  a  species  for  a  given  habitat.  In  effective- 
noneffective  rainfall  extremes,  as  in  the  means,  where  the  rainfall 
is  relatively  large,  the  non-effective  rainfall  is  relatively  small,  and  of 
a  consequence  there  is  an  increase  in  aridity  out  of  proportion  to  pos¬ 
sible  differences  in  total  precipitation. 

Such  considerations  as  briefly  presented  above  indicate  that  in 
comparing  one  region  with  another  on  the  basis  of  precipitation,  par¬ 
ticularly  if  the  regions  are  arid,  account  should  be  taken  of  the  amount 
of  rain  that  occurs  in  small  amounts,  but  which  nevertheless  may  be 
an  important  percentage  of  the  rainfall  taken  as  a  whole. 

MOISTURE  OF  THE  AIR. 

SEASONAL  VARIATION  IN  RELATIVE  HUMIDITY. 

The  annual  course  of  the  relative  humidity  follows  that  of  the 
rainfall,  and  is  not  necessarily  the  converse  of  that  of  the  temperature, 
as  frequently  is  the  case.  And,  according  to  Cox,1  “although  the 
amount  of  water-vapour  in  the  air  decreases  from  the  coast  inland 
the  north-easterly  and  easterly  winds  of  the  summer  months  convey 
to  the  high  veld  more  moisture  than  is  probably  present  on  the  same 
plane  at  the  coast.”  Possibly  an  analogous  condition  may  obtain 
in  winter  as  regards  the  western  coast  and  contiguous  interior  regions. 

The  annual  range  of  the  mean  relative  humidity  may  be  considerable. 
Thus  at  Johannesburg,  which  does  not  appear  to  be  extreme  in  this 
regard,  it  ranges  from  49  per  cent  in  September  to  75  per  cent  in  April, 
according  to  Knox.  During  the  months  of  August,  October,  and 
November,  the  mean  relative  humidity  is  54  to  58  per  cent;  in  the 
months  of  summer  it  is  60  to  70  per  cent;  in  autumn  it  is  71  to  75  per 
cent;  and  67  to  69  per  cent  in  June  and  July.  At  Table  Bay  the  mean 
minimum  is  67  per  cent  and  occurs  in  December  and  January,  and 
the  mean  maximum  is  in  May  to  August,  when  it  lies  between  80  and 
81  per  cent.  From  November  to  March  the  mean  relative  humidity 
is  between  67  and  69  per  cent.  It  is  74  per  cent  in  April  and  from 
73  to  77  per  cent  in  September  and  October. 

As  compared  to  the  conditions  which  obtain  at  Table  Bay,  the 
climate  at  Clanwilliam,  especially  the  relative  humidity,  has  certain 
points  of  interest.  Thus,  at  Clanwilliam,  which  lies  about  40  miles 

1  A  guide  to  botanical  survey  work.  Botanical  survey  of  South  Africa,  Mem.  No.  4,  p.  28. 
1922. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


39 


from  the  western  coast  and  about  130  miles  north  of  Cape  Town,  the 
annual  rainfall  is  8.57  inches,  of  which  about  79  per  cent  is  in  winter. 
The  mean  annual  relative  humidity  is  76  per  cent  as  opposed  to  the 
mean  annual  relative  humidity  at  Table  Bay  of  74  per  cent.  The 
annual  rainfall  at  the  latter  place,  which  also  occurs  mostly  in  the  cool 
seasons,  is  approximately  three  times  as  great  as  at  Clanwilliam. 
The  yearly  extremes  in  the  mean  relative  humidity  at  Clanwilliam 
are  from  61  per  cent  in  December  to  89  per  cent  in  August. 

In  the  eastern  portion  of  the  Central  Karroo,  at  Graaf  Reinet,  the 
mean  annual  relative  humidity  is  65  per  cent.  The  mean  monthly 
minimum  is  57  per  cent  and  occurs  in  July,  and  the  mean  maximum  is 
73  per  cent  in  February.  From  August  to  January  the  mean  relative 
humidity  ranges  from  60  to  68  per  cent  and  falls  to  64  per  cent  for  June. 
Graaf  Reinet  is  under  the  influence  of  the  summer  rains. 

At  Kimberley,  on  the  northern  border,  the  range  in  the  mean 
monthly  relative  humidity  is  from  46  per  cent  in  November  to  66  per 
cent  in  April  and  June.  The  mean  annual  relative  humidity  is  56  per 
cent. 

Calvinia,  in  the  upper  Karroo,  which  is  about  80  miles  from  the  west 
coast  and  about  200  miles  from  the  south  coast,  has  a  climatic  en¬ 
vironment  fairly  like  that  of  the  western  portion  of  the  Karroo.1 
Here  the  mean  relative  humidity  of  winter  lies  between  60  and  70  per 
cent  and  in  the  other  months  between  50  and  60  per  cent. 

At  Oudtshoorn,  in  the  Little  Karroo,  according  to  Dove,  the  mean 
annual  relative  humidity  is  76  per  cent.  The  mean  monthly  minimum 
is  in  February,  69  per  cent,  and  the  mean  monthly  maximum,  82  per 
cent,  is  in  May.  From  September  to  January  the  mean  monthly  rela¬ 
tive  humidity  runs  between  70  and  76  per  cent.  In  March  and  April 
it  is  76  and  79  per  cent,  and  May-August  it  is  82  to  80  per  cent. 

At  O’okiep,  in  the  Namaqualand  desert  province,  according  to 
Marloth,2  the  mean  annual  relative  humidity  is  48  per  cent;  the  mean 
monthly  minimum  is  40  per  cent,  November-December,  and  the 
mean  maximum  is  58  per  cent,  in  July.  From  April  to  August  the 
mean  monthly  humidity  ranges  between  52  and  58  per  cent,  and 
between  40  and  48  during  the  other  months. 

Data  on  the  relative  humidity  for  other  stations  of  the  Nama¬ 
qualand  desert  province  are  apparently  meager.  Dove  (Z.  c.)  gives  that 
at  Hope  Mine,  which  is  100  km.  southeast  of  Walfisch  Bay  and  80  km. 
from  the  coast,  as  44  per  cent  from  January  18  to  March  21.  At 
Ni-Guib,  from  June  to  September,  it  was  50  per  cent.  During  the 
middle  of  the  day  the  humidity  fell  to  31  per  cent  at  Hope  Mine  and 
26  per  cent  at  Ni-Guib. 


1  Das  Klima  des  aussertropischen  Siidafrika.  K.  Dove,  1888,  p.  51. 

2  Das  Kapland,  q.  v.,  p.  36. 


40 


FEATURES  OF  THE  VEGETATION  OF  THE 


WINDS. 

The  direction,  force,  frequency,  and  duration  of  winds  are  very 
important  elements  in  the  environment  of  plants,  particularly  in  arid 
or  semi-arid  regions.  Du  Toit,1  speaking  of  the  work  of  the  atmos¬ 
phere  as  a  geologic  agent,  with  especial  reference  to  wind  action,  says 
that  loess  is  being  formed  “  along  the  intermittent  rivers  and  on  the 
lee  sides  of  pans  in  northern  Cape  Province,  for  even  a  slight  breeze  is 
usually  sufficient  to  raise  clouds  of  fine  dust  from  the  dried  mud  which 
composes  the  floors  of  these  depressions.’ 7 

The  formation  of  sand-dunes  is  of  fairly  frequent  occurrence  in 
certain  regions  both  along  the  coast  and  in  the  interior.  They  con¬ 
stitute  a  prominent  character  of  the  physiography  along  the  coast 
at  Swakopmund  aud  southward  as  well  as  to  the  north.  Along  the 
Clanwilliam  coast  the  coarser  wind-blown  sand  makes  thick  deposits, 
obliterating  many  smaller  features  of  the  surface.  In  the  interior 
Du  Toit  says  that  the  accumulation  of  the  sand  into  dunes  in  places 
retains  a  height  of  about  100  feet  and  there  is  a  long  succession  of 
sandhills.  He  mentions  how  the  channel  of  the  Molopo  River,  which 
drains  a  portion  of  the  Kalahari,  has  been  choked  by  sand  so  that  on 
the  rare  occurrence  of  water  in  the  river  it  is  deflected  from  its  proper 
course  to  make  its  way  through  sandhills,  where  it  disappears.  Along 
river  banks  there  frequently  occur  sandy  stretches  which  derive  the 
sand  from  river-beds,  when  dry,  through  wind  action.  Some  of  these 
accumulations  attain  a  height  of  40  feet.  In  southern  Algeria  sand 
is  strewn  in  a  similar  manner  along  and  near  stream- ways,  forming  a 
mulch  which  serves  to  better  conserve  the  small  amount  of  moisture 
in  the  soil,  as  is  evidenced  by  the  relatively  abundant  plant  population 
of  such  areas.2 

The  winds,  especially  in  arid  regions,  are  often  important  agents  of 
erosion  through  the  polishing  and  grinding  action  of  the  sand  which 
they  carry.  In  the  Libyan  Desert  pebbles  of  good  size  are  thus  trans¬ 
ported.  Often  the  rock  surfaces  exposed  to  the  erosive  action  of  the 
sand-carrying  winds  are  polished  or  worn  away,  especially  near  the 
ground,  leaving  the  upper  or  harder  portions.  The  exposed  surfaces 
of  small  stones,  also,  may  be  polished  or  flattened,  as  in  the  case  of 
the  “gibbers’’  of  vast  gibber-plains  in  northern  South  Australia.3 * 
Although  similar  action  must  be  in  progress  in  South  Africa,  it  is  not 
of  sufficient  importance  to  be  mentioned  by  Du  Toit. 

The  direction  of  the  prevailing  winds  in  different  portions  of  southern 
Africa  changes  with  the  seasons,  although  in  certain  stations  the  winds 
appear  to  be  fairly  constant,  on  the  whole,  as  to  the  general  direction. 

1  Physical  Geography  of  South  African  Schools,  Cambridge,  p.  67,  1921. 

2  Botanical  features  of  the  Algerian  Sahara.  W.  A.  Cannon.  Carnegie  Inst.  Wash.  Pub. 
No.  178,  1913. 

3  Plant  habits  and  habitats  in  the  arid  portions  of  South  Australia.  W.  A.  Cannon.  Carnegie 

Inst.  Wash.  Pub.  No.  308,  1921. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


41 


In  portions  of  Southwest  Africa,  as  at  Omaruru,  according  to  Dove 
(I.  c.),  in  the  warm  seasons  the  winds  are  mainly  from  the  northeast 
quadrant,  while  in  the  cool  seasons  they  are  mainly  from  the  south¬ 
west-northwest  quadrant.  The  former  bring  the  summer  rains. 

At  Graaf  Reinet,  in  the  Central  Karroo,  out  of  696  observations 
it  was  found  that  in  the  warm  season  the  winds  came  mainly  from  the 
southeast  and  in  the  cool  season  mainly  from  the  northwest.  At 
Aliwal  North,  in  the  warmer  seasons  the  winds  are  mainly  south¬ 
easterly,  while  winter  appears  to  be  largely  a  season  of  calm.  At 
Grahamstown  the  prevailing  winds,  both  of  the  warmer  and  of  the 
colder  seasons,  are  from  the  southwest,  although  in  the  warmer 
seasons  there  is  a  large  proportion  of  southeasterly  winds,  and  in  the 
colder  seasons  a  large  proportion  of  winds  from  the  west.  Under 
proper  conditions  the  moisture  in  such  winds  is  precipitated  as  rain. 
Where  such  is  not  the  case  they  nevertheless  may  directly  affect 
vegetation  through  raising  the  humidity,  as  was  frequently  observed 
in  the  Karroo  in  early  spring.  On  the  Namib,  too,  the  sea-breeze 
brings  inland  heavy  fog,  and  at  Swakopmund  and  elsewhere  on  the 
Namib  the  moisture  of  such  winds  may  be  deposited  like  dew.  Thus 
Pearson1  says: 

“Other  features  of  this  most  interesting  desert-region,  the  Namib,  which 
force  themselves  upon  the  notice  of  the  traveller  are  the  mirage  and  the 
night  fogs .  The  night  fogs  in  so  arid  a  region  are  hardly  less  remark¬ 

able.  About  8  o'clock  in  the  evening  of  January  24,  as  we  crossed  the  dry 
bed  of  Tubas  River,  a  cloud  appeared  in  the  west.  The  stars  were  gradually 
obliterated,  and  by  10  o'clock  we  were  shrouded  in  a  cold  ‘Scotch  mist.' 
After  sleeping  on  the  ground  from  midnight  until  4  o'clock  on  the  following 
morning  I  was  able  to  wring  the  water  out  of  my  top-covering,  a  woollen  rug. 
As  soon  as  daylight  came,  the  ground  was  seen  to  be  discoloured  by  the 
moisture  absorbed,  and  the  plants  were  copiously  sprinkled  with  dew.  At 
7  a.  m.,  on  January  30,  the  water  was  dripping  from  the  branches  of  the 
Tamarisks  in  the  Khan  valley  at  Haikamchab.'' 

The  fog  drifts  inland  with  the  sea-breeze  to  a  depth  of  30  miles, 
more  or  less. 

When  the  east  wind  blows  on  the  Namib  there  is  neither  fog  nor 
dew,  and  intensely  arid  conditions  obtain.  According  to  Knox,  winds 
of  this  kind  are  characteristic  of  the  west  coast  to  the  north  of  Clan- 
william. 

MacDougal 2  states  that  in  portions  of  the  Libyan  Desert  the  lack 
of  vegetation  may  not — 

“ultimately  be  due  to  lack  of  water  but  to  wind-action.  Plants  are  found  in 
innumerable  places  in  which  the  supply  is  no  greater  than  that  of  the  bare 
areas,  but  in  exposed  places  the  surface  layers  of  sand  and  gravel  are  shifted 
about,  exercising  a  corrosive  action  that  is  destructive  to  plants  and  highly 

1  Some  notes  on  a  journey  from  Walfisch  Bay  to  Windhuk.  H.  H.  W.  Pearson.  Bui.  Misc. 
Information,  No.  9,  1907,  Roy.  Bot.  Gar.,  Kew,  p.  349. 

2  The  deserts  of  western  Egypt.  Plant  World,  vol.  16,  1913,  p.  303. 


42 


FEATURES  OF  THE  VEGETATION  OF  THE 


important  in  determining  the  contours  of  hills  and  the  surface  of  rocks. 
Highly  specialized  desert  species  might  survive  the  aridity,  but  the  shifting 
substratum  does  not  permit  them  to  attain  maturity,  a  condition  which 
would  affect  both  the  tender,  rapidly  developing  annual  and  the  slowly 
growing  leathery  xerophytes.” 

The  character  of  the  winds  referred  to  in  the  preceding  paragraphs 
are  of  a  general  nature,  extending  over  wide  areas,  and  may  largely 
be  associated  with  far-extending  atmospheric  disturbances,  such  as 
general  rains.  They,  moreover,  may  be,  and  frequently  are,  of  con¬ 
siderable  force,  whether  moisture-bearing  or  otherwise.  There  is, 
however,  quite  another  type  which  is  also  of  importance  to  plant  life 
in  arid  regions,  and  which  are  not  referred  to  in  meteorological  reports. 
Such  are  purely  local  winds,  which  may  be  extremely  variable  as  to 
direction,  duration,  force,  and  capacity  of  giving  out  or  of  taking  up 
moisture.  Often  these  are  little  more  than  convection  air-currents, 
but  which  notwithstanding  this  may  be  important  in  modifying  the 
relative  humidity  and  the  temperature  and  in  this  manner  more 
especially  directly  affect  plant  life.  Whatever  may  be  the  origin 
of  such  winds,  they  are  practically  always  to  be  found  in  arid  regions, 
where  they  are  especially  noticeable  because  of  the  paucity  of  cover. 
At  Beaufort  West  and  Matjesfontein  it  was  often  noted  that  how¬ 
ever  calm  the  veld  was  in  early  morning  the  breezes  began  with  the 
rising  of  the  sun  and  seldom  stopped  through  the  day.  When  such 
winds  came  from  higher  and  more  humid  elevations  they  notably 
increased  the  relative  humidity.  This  was  noticed  at  Matjesfontein 
in  September,  in  connection  with  winds  from  the  south  and  southeast, 
and  which  were  of  local  origin.  When,  however,  they  came  from  the 
opposite  direction,  the  humidity  of  the  air  was  maintained  very  low, 
so  that  in  working  with  cobalt-chloride  papers  in  studying  the  trans¬ 
piration  of  certain  plants,  the  papers  remained  dark  blue  even  when 
exposed  to  the  air,  when  under  other  conditions  they  would  quickly 
assume  a  pink  color.  Such  winds  also  were  found  to  have  a  note¬ 
worthy  effect  on  the  rate  of  evaporation,  as  shown  by  the  readings 
of  the  atmometers,  circumstances  that  will  be  spoken  of  in  another 
place. 

EVAPORATION. 

Investigations  on  evaporation  in  southern  Africa  have  proceeded 
along  two  lines.  They  either  have  had  to  do  with  the  amount  of 
water  lost  from  a  free  water-surface  or  the  amount  lost  from  the  moist 
surface  of  a  porous  clay  cup  or  atmometer.  The  former  extend  over 
a  fairly  long  period  and  for  the  most  part  have  been  carried  on  by  the 
meteorological  bureau  of  the  government,  while  the  latter,  on  the 
other  hand,  begun  by  volunteer  observers,  are  being  taken  up  by  the 
Botanical  Survey  of  South  Africa,  and  cover  but  a  short  period  of 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


43 


time.  Their  immediate  aim  is  to  define,  so  far  as  possible,  the  evap¬ 
orating  power  of  the  air  as  an  environmental  factor  of  plants. 

At  Kimberley,  according  to  Twigg1  the  evaporimeter  consisted 
of  a  wrought-iron  tank  4  by  4  by  4  feet  in  size,  sunk  in  the  ground 
within  1  inch  of  the  surface  and  kept  full  of  water.  The  mean  evap¬ 
oration  was  86.68  inches  and  the  highest  was  104.39  inches.  The 
least  monthly  mean,  4.3  inches,  was  in  June,  and  the  greatest,  12.1 
inches,  in  December.  From  April  to  August,  inclusive,  the  monthly 
mean  was  between  4.3  inches  and  6.4  inches.  From  October  to 
January  it  was  between  10.8  inches  (October),  and  12.1  inches.  The 
rate  decreased  month  by  month  from  January  to  June,  when  it  in¬ 
creased  steadily  until  the  end  of  the  year. 

With  the  use  of  the  type  of  apparatus  above  referred  to,  it  was 
found  that  in  1894  the  evaporation  was  91.26  inches,  and  in  1895 
it  was  101.84  inches.  From  1894  until  1900  another  type  of  free 
water-surface  evaporimeter  was  employed  at  Kimberley  which  gave 
a  mean  of  90.1  inches  for  this  period.  In  1894,  by  the  later  apparatus, 
it  was  77.93  inches,  and  in  1895,  88.43  inches,  which  will  be  seen  to 
be  somewhat  less  for  the  two  years  than  was  found  to  be  the  case 
with  the  earlier  type  of  evaporimeter  employed. 

At  Kimberley,  in  the  year  1900,  a  comparative  study  was  carried 
out  of  four  different  types  of  evaporimeters,  the  results  of  which  need 
only  be  referred  to  in  this  place.  The  use  of  an  8-inch  copper  pan 
gave  90.82  inches,  a  screened  tub  gave  61.98  inches,  a  tank  gave  55.21 
inches,  and  a  Piche  tube  gave  82.83  inches. 

Cox  gives  the  annual  evaporation  at  Johannesburg,  the  sum  of 
the  monthly  means,  as  74.67  inches,  and  at  Cape  Town  as  78.57  inches. 
The  least  monthly  evaporation  at  the  latter  is  4.35  inches  and  at  the 
former  2.48  inches,  both  in  June.  At  Johannesburg  the  highest 
monthly  mean  is  8.41  in  October  and  at  Cape  Town  10.39  in  January. 
The  seasonal  fluctuation  of  evaporation  is  greater  also  at  Cape  Town 
than  at  Johannesburg.  Thus,  in  summer,  at  Cape  Town,  the  evap¬ 
oration  is  30.2  and  at  Johannesburg  it  is  19.5  inches,  and  in  autumn 
for  the  two  stations  respectively  it  is  18.01  and  15.2  inches.  In  winter 
it  is  9.6  and  15.86  inches.  In  spring  the  evaporation  at  Cape  Town 
is  20.7  and  at  Johannesburg  it  is  23.95  inches.  At  Cape  Town  77 
per  cent  of  the  rainfall  is  in  winter  and  at  Johannesburg  87  per  cent 
of  the  rainfall  is  in  summer.  It  will  be  of  interest  to  compare 
the  above  seasonal  evaporation  means  at  Cape  Town  and  Johannes¬ 
burg  with  that  at  Kimberley.  Thus,  at  Kimberley  the  mean  evap¬ 
oration  for  summer,  autumn,  winter,  and  spring  is  32,  19.5,  15.5,  and 
30.1  inches.  At  Kimberley  78  per  cent  of  the  rainfall  is  in  summer. 
The  mean  summer  temperature  at  Kimberley  is  75.86°  F.,  at  Cape 

1  Quarterly  Journ.  Roy.  Soc.  Met.  Sci.,  vol.  22,  p.  166,  referred  to  by  Sutton.  Trans.  So.  African 
Phil.  Soc.,  vol.  14,  pt.  1,  1902. 


44 


FEATURES  OF  THE  VEGETATION  OF  THE 


Town  it  is  68.2°,  and  at  Johannesburg  it  is  63.46°  F.  There  appears 
thus  to  be  a  direct  relation  between  the  summer  temperature  at  these 
three  stations  and  the  mean  evaporation  for  the  same  season.  This 
is  of  some  interest,  in  view  of  the  fact  that  summer  is  the  season  of 
greatest  rainfall  at  Kimberley  and  Johannesburg  and  the  least  rainfall 
at  Cape  Town,  as  above  stated. 

RATIO  OF  RAINFALL  TO  EVAPORATION  (P/E). 

The  precipitation-evaporation  ratio  is  a  convenient  way  of  com¬ 
paring  stations  of  different  regions,  as  well  as  different  seasons  at  one 
station,  as  to  the  relation  of  rainfall  to  water-lost  through  the  evapo¬ 
rating  power  of  the  air,  for  the  station  or  for  the  seasons.  The  ratio 
is  said  by  Livingston  and  Shreve1  to  be  “the  nearest  approach  that  is 
yet  possible  toward  an  ideal  index  of  the  external  moisture  relations 
of  plants, n  and  as  a  general  expression  of  the  amount  of  water  avail¬ 
able  to  vegetation  it  is  of  great  use. 

The  data  at  hand  do  not  permit  a  satisfactory  discussion  of  the 
P/E  for  South  Africa,  so  that  this  index  can  not  be  charted  as  isor- 
ropic  lines  for  geographical  uses.  However,  some  account  can  be 
presented  of  the  precipitation-evaporation  for  Cape  Town,  Kimberley, 
and  Johannesburg,  representing  the  region  of  winter  and  of  summer 
rainfall.  The  P/E  of  the  three  stations  is  shown  graphically  in 
figure  8.  The  high  evaporation-rate  in  summer  at  Cape  Town,  as 
compared  to  the  rainfall  for  the  season,  forms  a  marked  contrast  to 
the  condition  obtaining  at  Kimberley,  and  especially  at  Johannesburg, 
where,  owing  to  the  large  summer  precipitation,  the  evaporation-rate 
is  relatively  low.  At  Johannesburg,  in  January,  the  rainfall  equals 
the  evaporation,  but  in  the  balance  of  the  year  it  is  less.  At  Kimberley 
it  is  always  less  than  the  evaporation,  and  at  Cape  Town,  for  the  three 
winter  months,  the  rainfall  is  much  in  excess.  The  violent  seasonal 
contrast  in  the  external  moisture  relations  of  the  plants  at  each  of  the 
three  stations,  particularly  at  Cape  Town  and  at  Johannesburg,  are  also 
strikingly  shown  in  the  graph. 

ATMOMETRY  IN  SOUTHERN  AFRICA. 

The  type  of  evaporimeter  used  in  obtaining  the  results  mentioned 
in  the  preceding  paragraphs  does  not  appear  to  be  well  suited  for 
intensive  studies  on  evaporation  as  one  of  the  important  physical 
factors  of  the  environment,  and  it  was  decided  to  introduce  the 
spherical  atmometer  as  developed  by  Livingston.2  Accordingly 

1  The  distribution  of  vegetation  in  the  United  States,  as  related  to  climatic  conditions.  Car¬ 
negie  Inst.  Wash.  Pub.  No.  284,  p.  326,  1921. 

2  Atmometery  and  the  atmometer.  Plant  World,  vol.  18,  p.  148,  1915. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


45 


atmometers  were  placed  at  certain  representative  stations,  named 
below,  and  were  read  by  volunteer  observers.1 

The  work  here  reported  must  be  considered  introductory,  merely, 
and  conclusions  based  on  it  are  tentative.  The  results  to  be  given 
are  taken  directly  from  the  field-books,  for  the  reason  that  at  the  time 
of  the  visit  there  was  no  means  of  restandardizing  the  instruments, 
and  further,  because  for  the  most  part  they  deal  with  relatively  short 
periods.  As  time  went  on,  however,  as  the  accompanying  note 
intimates,  there  was  increasing  need  of  the  restandardization  in  most 
instances.  The  records  for  Grahamstown  and  Matjesfontein,  how¬ 
ever,  which  are  of  much  importance,  are  apparently  satisfactory. 

In  the  preliminary  studies  the  atmometers  were  placed  and  read 
at  the  following  stations:  Messina  and  Pretoria,  Transvaal;  Pieter¬ 
maritzburg,  Natal;  Grahamstown,  Cape  Town,  Beaufort  West,  and 
Matjesfontein,  Cape  of  Good  Hope;  Swakopmund,  Protectorate  of 
Southwest  Africa.  Messina  is  situated  in  the  Low  Veld,  Pretoria  on 
the  northern  edge  of  the  High  Veld,  Pietermaritzburg  in  the  Eastern 
Grass  Veld,  Grahamstown  fairly  between  the  latter  and  the  Cape 
region,  Beaufort  West  and  Matjesfontein  in  the  Karroo  Province, 
and  Swakopmund  in  Namaqualand  Desert  Province. 

NATIONAL  BOTANIC  GARDENS. 

At  the  National  Botanical  Gardens,  Kirstenbosch,  the  atmometer 
was  placed  on  the  north  slope,  6  feet  above  the  ground,  at  an  altitude 
of  about  350  feet  above  sea-level,  with  other  meteorological  instru¬ 
ments. 

The  records  at  hand  are  for  one  year.  Some  difficulty  was  experi¬ 
enced  from  the  flooding  of  the  reservoir,  owing  to  the  faulty  function¬ 
ing  of  the  valves  used,  so  that  the  record  is  not  complete.  Table  7 
gives  the  weekly  evaporation  for  the  period,  less  the  gaps  referred  to. 
It  is  given  in  its  entirety  for  the  reason  that  it  is  the  only  long  atmom¬ 
eter  record  at  hand  covering  the  region  with  winter  rains. 

Owing  to  the  lack  of  completeness,  without  reference  to  possible 
instrumental  errors,  only  general  conclusions  can  be  drawn  from  the 
gardens’  record.  So  far  as  can  be  told,  the  monthly  evaporation 
for  the  period  in  question  varied  from  about  572  c.  c.  in  September 
to  about  1,012  c.  c.  in  February,  although  the  minimum  amount 
might  well  have  been  less.  The  entire  evaporation  for  spring  was 
2,373  as  compared  to  an  evaporation  of  2,872  for  the  following  summer. 

1  Such  was  the  status  while  the  writer  was  in  South  Africa.  Upon  his  leaving,  however,  the 
Botanical  Survey  took  over  the  work  and  at  present  it  is  being  conducted  for  the  Survey  by  Mr. 
R.  D.  Aitken,  Natal  University  College,  Pietermaritzburg.  Mr.  Aitken  restandardized  the 
atmometers  above  referred  to  about  November  25,  1922,  or  before  that  date,  and  has  kindly 
supplied  revised  coefficients  for  them  as  follows:  Grahamstown,  0.90  to  0.88;  National  Botanical 
Gardens,  Kirstenbosch,  very  variable,  0.90  to  1.51;  Matjesfontein,  0.91;  Pretoria,  badly  altered, 
1.32  to  1.62. 


46 


FEATURES  OF  THE  VEGETATION  OF  THE 


About  2,131  c.  c.  of  water-loss  occurred  in  autumn  and  approximately 
one-half  that  amount  in  winter.  The  seasonal  range  in  evaporation 
at  Kirstenbosch  is,  therefore,  very  considerable.  It  is  apparent  that 
the  amount  of  evaporation  in  the  warm  seasons,  for  the  period  con- 


Table  7. — Weekly  evaporation  at  the  National  Botanic  Gardens ,  Kirstenbosch,  August  15, 

1921,  to  September  29,  1922. 


Period  of  observation. 

Water  loss. 

Period  of  observation. 

Water  loss. 

1921. 

c.c. 

1922. 

c.c. 

Aug.  15  to  22 . 

90 

Feb.  11  to  17 . 

218 

Aug.  23  to  29 . 

192 

Feb.  18  to  24 . 

215 

*  *  *  *  a 

Feb.  25  to  Mar.  4 . 

256 

Sept.  16  to  24 . 

133 

Mar.  5  to  11 . 

231 

*  *  *  * 

Mar.  12  to  17 . 

188 

Oct.  14  to  21 . 

168 

Mar.  18  to  24 . 

124 

Oct.  22  to  28 . 

203 

Mar.  25  to  Apr.  1 . 

115 

Oct.  29  to  Nov.  5 . 

250 

Apr.  2  to  8 . 

330 

Nov.  6  to  11 . 

250 

Apr.  9  to  14 . 

41 

Nov.  12  to  18 . 

157 

Apr.  15  to  21 . 

166 

Nov.  19  to  25 . 

220 

Apr.  22  to  28 . 

154 

Nov.  26  to  Dec.  2 . 

237 

Apr.  29  to  May  13 . 

315 

Dec.  3  to  9 . 

298 

May  14  to  20 . 

174 

Dec.  10  to  17 . 

180 

May  21  to  26 . 

37 

Dec.  18  to  24 . 

251 

May  27  to  June  2 . 

122 

Dec.  25  to  30 . 

141 

*  *  *  * 

July  7  to  14 . 

168 

1922. 

July  15  to  21 . 

239 

Dec.  30  to  Jan.  6 . 

334 

July  22  to  28 . 

87 

Jan.  7  to  13 . 

167 

*  *  *  * 

Jan.  14  to  20 . 

105 

Sept.  9  to  16 . 

58 

Jan.  21  to  27 . 

215 

Sept.  17  to  23 . 

114 

Jan.  28  to  Feb.  3 . 

157 

Sept.  24  to  29 . 

133 

Feb.  4  to  10 . 

242 

a  The  asterisks  denote  lack  of  record  from  flooding  of  reservoir. 


Table  8. — Current  and  normal  precipitation-evaporation  ratios  (P/E)  at  the  National  Botan¬ 
ical  Gardens,  1921-22,  with  current  and  normal  rainfall  for  Wynberg. 


P/E 

1921-22. 

P/E 

normal. 

Rainfall 

Wynberg, 

1921-22. 

Rainfall 

Wynberg, 

normal. 

Evapora¬ 

tion. 

inches. 

inches. 

c.  c. 

June . 

0.0284 

0.0160 

11.31 

6.47 

398 

July . 

.0100 

.0153 

5.26 

7.70 

508 

Aug . 

.0403 

.0403 

6.67 

a  6.67 

163 

Sept . 

.0066 

.0075 

3.82 

4.31 

572 

Oct . 

.0034 

.0027 

2.58 

2.03 

742 

Nov . 

.0009 

.0013 

0.86 

1.26 

959 

Dec . 

.0011 

.0010 

1.06 

0.95 

931 

Jan . 

.0018 

.0005 

1.71 

0.48 

929 

Feb . 

.0006 

.0005 

0.63 

0.53 

1,012 

Mar . 

.0002 

.0024 

0.13 

1.75 

714 

Apr . 

.0021 

.0034 

1.83 

2.93 

861 

May . 

.0062 

.0110 

2.99 

5.52 

476 

a  Normal  rainfall  for  August. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


47 


sidered,  was  approximately  62  per  cent  of  the  whole,  or  not  far  from 
twice  that  of  the  cool  seasons. 

It  would  be  desirable  to  present  the  precipitation-evaporation 
ratio  for  the  gardens  for  the  seasons  and  the  months  covered  by  the 
records,  only  unfortunately,  as  this  is  written,  there  are  no  Kirsten- 
bosch  rainfall  records  at  hand.  However,  an  approximation  will 
be  made  by  substituting  the  records  of  precipitation  for  Wynberg, 
which  is  about  3  miles  distant.  In  this  the  normal  rainfall  at  Wynberg, 
as  well  as  that  for  the  particular  months  in  question,  will  be  used. 
A  summary  of  the  results  are  given  in  table  8. 

It  will  be  seen  that  the  normal  monthly  values  of  the  P/E  and 
those  for  the  particular  month  are  very  unlike.  The  normal  extremes 
are  0.0005  and  0.016,  while  the  current  extremes  are  0.00018  and 
0.0284.  The  tentative  mean  monthly  indices  obtained  by  dividing 
the  seasonal  total  by  3  are:  summer,  0.0011;  autumn,  0.0028;  winter, 
0.014;  spring,  0.0036.  The  order  of  the  relative  aridity  of  the  seasons 
are,  therefore,  winter,  spring,  autumn,  and  summer. 

GRAHAMSTOWN. 

At  Grahamstown  the  atmometer  records  at  hand  are  from  June 
23  to  November  3,  1921.  The  atmometer  was  placed  in  a  yard  some¬ 
what  sheltered  from  winds,  but  exposed  to  the  sun  throughout  the  day. 
The  altitude  is  about  1,350  feet  above  sea-level.  The  period  of 
observation  includes  all  of  winter  and  the  first  month  of  spring. 

As  shown  elsewhere,  Grahamstown  is  fairly  intermediate  between 
the  region  of  summer  rains  and  the  region  of  winter  rains,  with  the 
effect  that  the  precipitation  is  not  markedly  periodic,  but  approaches 
equal  distribution  through  the  year.  In  winter  39  per  cent  of  the 
annual  rainfall  occurs.  It  increases  to  a  primary  maximum  in  spring 
and  there  is  a  secondary  maximum  in  autumn. 

It  seems  not  improbable  that  the  general  distribution  of  the  rain¬ 
fall  will  be  found  to  be  reflected  in  the  precipitation-evaporation  ratio 
for  the  station,  as  is  suggested  by  the  meager  data  at  hand.  The 
actual  water-loss,  for  a  week,  ranged  between  119.5  c.  c.  for  the  week 
ended  September  8  and  261.2  c.  c.  for  the  week  ended  June  30.  The 
total  monthly  evaporation  was  as  follows:  July,  920  c.  c.;  August, 
960  c.  c. ;  September,  1,207  c.  c. ;  October,  808  c.  c.  The  normal  annual 
rainfall  for  July  is  0.67  inch,  for  August  1.08  inches,  for  September 
3.28  inches,  and  for  October  3.1  inches.  From  this  it  will  be  seen  that 
the  normal  P/E  ratio  for  July  is  0.00072;  for  August  it  is  0.0010;  for 
September  it  is  0.0027,  and  for  October  it  is  0.0038. 

The  normal  rainfall  for  July,  August,  and  September  is  greater  at 
Wynberg  than  at  Grahamstown,  but  the  October  rainfall  at  Grahams¬ 
town  exceeds  that  at  Wynberg.  The  relative  aridity  for  July,  August, 
and  September  is  apparently  greater  at  Grahamstown,  but  the  con- 


48 


FEATURES  OF  THE  VEGETATION  OF  THE 


verse  appears  to  be  true  for  October,  as  would  be  expected  from  the 
rainfall  records  and  as  is  indicated  by  the  P/E  ratios.  A  com¬ 
parison  of  the  precipitation-evaporation  indices  based  on  the  current 
rainfall  might  give  even  more  striking  results. 

PIETERMARITZBURG. 

At  Pietermaritzburg,  altitude  2,218  feet  above  the  sea,  the  at- 
mometer  was  placed  on  the  roof  of  University  College.  The  records 
at  hand  are  from  the  week  ending  August  16,  1921,  to  the  week  ending 
February  28,  1922.  They  thus  include  the  last  month  of  winter  and 
spring,  and  nearly  two  months  of  summer. 

Table  9. — Precipitation-evaporation  ratios  (P/E)  at  Pietermaritzburg, 

October  1921  to  February  1922. 

[Normal  rainfall  and  current  rainfall  in  inches;  evaporation  in  cubic  centimeters.] 


Evapora¬ 

tion. 

Rainfall, 

normal. 

P/E, 

normal. 

Rainfall, 

1921-22. 

P/E, 

1921-22. 

c.  c. 

inches. 

inches. 

Oct . 

1,077 

3.14 

0.0029 

3.94 

0.0036 

Nov . 

884 

4.08 

.0046 

6.34 

.0071 

Dec . 

1,016 

5.04 

.0049 

9.05 

.0088 

Jan . 

1,313 

5.21 

.0039 

2.41 

.0018 

Feb . 

1,235 

5.06 

.0040 

2.33 

.0018 

The  weekly  amount  of  evaporation  ranged  from  168  c.  c.,  for  the 
week  ending  November  29  to  more  than  330  c.  c.,  for  the  week  ending 
September  20.  The  monthly  totals  for  the  period  are:  October,  1,077 
c.  c.;  November,  884  c.  c.;  December,  1,016  c.  c.;  January,  1,313  c.  c.; 
and  February,  1,235  c.  c. 

The  normal  precipitation-evaporation  ratio  and  the  P/E  are  given 
in  table  9,  from  which  it  will  appear  that  the  least  normal  P/E  ratio 
is  0.0029  for  October,  and  that  the  greatest  is  0.0049  and  is  for  Decem¬ 
ber.  The  current  ratios,  on  the  other  hand,  are  least  in  January  and 
February  and  greatest  in  December,  and  on  the  whole  correspond  but 
poorly  with  the  normal  ratios.  The  difference  is  clearly  related  to  the 
departure  from  the  normal  of  the  rainfall  for  the  months  in  question. 

MATJESFONTEIN. 

At  Matjesfontein,  altitude  2,953  feet,  the  atmometer  was  placed  on 
a  fence  in  a  yard  which  was  somewhat  protected  from  the  southerly 
winds.  It  was  exposed  to  the  sun  throughout  the  day. 

The  records  at  hand  are  from  the  week  ending  September  12,  1921, 
to  the  week  ending  March  28,  1922.  The  evaporation  data,  there¬ 
fore,  relate  to  most  of  the  spring,  all  of  the  summer,  and  to  one  of  the 
autumn  months. 

A  summary  of  the  evaporation-precipitation  ratios  for  the  period 
of  the  observations  is  given  in  table  10. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


49 


The  Matjesfontein  record,  although  short,  is  of  interest  because  it 
suggests  the  extremely  variable  P/E  values  obtaining  in  an  arid  region. 
There  was  little  or  no  precipitation  in  September,  October,  November, 
and  February  1921-22,  but  2.75  inches  occurred  December  25-28. 
Assuming,  for  purposes  of  comparison,  that  the  non-effective  amount 
for  November  occurred  in  October  and  February  as  well,  we  obtain 
extremely  low  ratios  for  those  months.  The  high  evaporation-rate 
for  December  is  associated  with  the  want  of  rain  during  24  days  of 
the  month,  at  the  time  of  mounting  summer  temperature. 

The  annual  course  of  the  P/E  for  Matjesfontein  will  thus  probably 
be  found  to  exhibit  an  extremely  wide  range.  The  ratio,  as  above 
appears,  is  very  low  for  summer,  and  with  63  per  cent  of  the  pre¬ 
cipitation  occurring  in  winter,  it  can  be  expected  to  be  correspondingly 
high  for  that  season. 

Table  10. — Normal  and  current  precipitation-evaporation  ratios  at  Matjesfontein, 
October  1921  to  February  1922,  with  current  and  normal  rainfall. 


Evapora¬ 

tion. 

Normal 

precipi¬ 

tation. 

Normal 

P/E. 

Current 

precipi¬ 

tation. 

Current 

P/E. 

c.  c. 

inch. 

inches. 

Oct . 

1,605 

0.58 

0.000360 

a0.11 

® 0 . 000068 

Nov . 

1,989 

.60 

.000310 

.11 

.00005 

Dec . 

2,297 

.19 

.000082 

2.75 

.00119 

Jan . 

1,635 

.28 

.000016 

1.55 

.00093 

Feb . 

1,431 

.69 

.000480 

°  .11 

. 00005 

a  No  rainfall  is  reported,  and  for  comparative  purposes  0.11  inch  is  assumed 
to  have  occurred,  as  in  the  following  month. 

BEAUFORT  WEST. 


At  Beaufort  West,  altitude  2,792  feet  above  the  sea,  the  atmometer 
record  at  hand  runs  from  the  week  ending  August  22  to  the  week 
ending  December  5,  1921. 1  Up  to  November  the  atmometer  was 
situated  in  a  garden,  somewhat  sheltered  from  the  wind,  but  exposed 
to  the  sun  most  of  the  day.  In  November  and  December,  however, 
it  was  removed  and  placed  on  the  roof  of  an  outbuilding,  where  it  was 
exposed  to  the  wind  as  well  as  to  the  sun. 

At  Beaufort  West  the  annual  rainfall  is  9.56  inches,  of  which  about 
34  per  cent  occurs  in  winter  and  66  per  cent  in  summer.  From  this 
it  will  be  seen  that  the  station  is  largely  under  the  influence  of  the 
rains  of  the  warm  seasons,  especially  of  summer. 

1  Since  the  above  was  written  the  atmometer  records  for  Beaufort  West,  covering  the  period 
between  Janury  5  and  May  29,  1922,  have  been  received.  The  total  evaporation  for  February 
was  568  c.  c.  and  for  March  400  c.  c.  The  rainfall  for  the  two  months  was  0.46  and  1.56  inches, 
respectively.  That  for  March  was  normal,  but  that  for  February  was  0.80  inch  below.  The 
normal  P/E  for  February  is  0.0022  and  the  current  for  the  month  is  0.000S9.  The  normal  and 
current  P/E  for  March  is  0.0039. 


50 


FEATURES  OF  THE  VEGETATION  OF  THE 


During  the  period  given  above  the  atmometer  was  read  daily  and 
daily  maximum  and  daily  minimum  temperatures  of  the  air  were 
recorded.  In  August  a  minimum  of  31°  F.  was  observed,  which  was 
the  lowest  for  the  period,  and  in  November  an  absolute  maximum  of 
102°  F.  was  recorded.  The  greatest  weekly  variation  in  temperature 
occurred  for  the  week  ending  October  16,  when  the  maximum  was  98° 
and  the  minimum  43°.  The  greatest  daily  variation  was  44°  F.  which 
was  noted  on  two  occasions,  November  23  and  September  30.  The 
daily  variation  in  temperature  was  especially  large  during  fair  weather 
and  clear  skies;  when,  however,  rain  was  reported,  as,  for  instance, 
on  November  9,  it  was  small,  the  maximum  being  65°  and  the  mini¬ 
mum  54°  F.  All  of  these  conditions,  as  will  be  seen  below,  directly 
affected  the  rate  of  evaporation. 

The  water-loss  from  the  atmometer  in  September  was  1,414  c.  c. 
and  in  October  1,381  c.  c.  During  these  two  months,  as  above  re¬ 
marked,  the  atmometer  was  situated  in  a  garden,  not  irrigated  at  the 
time,  and  somewhat  protected  from  the  wind.  In  November,  when 
it  was  placed  on  a  roof,  the  evaporation  was  1,350  c.  c.  The  least 
weekly  evaporation  was  181  c.  c.  and  the  greatest  was  615  c.  c.,  both  of 
which  occurred  during  the  month  of  September,  the  former  for  the 
week  ending  the  12th  and  the  latter  for  the  week  ending  the  19th. 
In  November  the  extremes  were  165  and  429  c.  c.  According  to  the 
data  at  hand,  the  greatest  water-loss  occurring  in  one  day  was  on 
October  26-27,  when  an  evaporation  of  69  c.  c.  was  recorded.  On 
October  26  the  maximum  temperature  was  96°  and  the  minimum  was 
59°,  and  on  the  27th  the  maximum  was  87°  and  the  minimum  60°  F. 
The  least  daily  evaporation,  10  c.  c.,  was  recorded  on  September  3. 
The  range  of  temperature  for  the  day  was  from  43°  to  67°  F. 

During  the  last  week  of  October  and  the  second  week  of  November 
the  atmometer  readings  are  of  interest  from  the  fact  that  in  the  first 
instance  the  weather  was  fair  and  the  temperature  high,  with  favor¬ 
able  conditions  for  a  high  rate  of  evaporation,  as  was  experienced  as 
above  noted,  and  in  the  second  instance,  the  week  in  November, 
there  was  some  rain.  The  total  evaporation  for  the  last  week  in 
October  was  351  c.  c.  and  for  the  second  week  of  November  was  165.4 
c.  c.  The  daily  extremes  in  the  amount  of  water  lost  during  the  last 
week  in  October  were  30  and  69  c.  c.  and  during  the  second  week  in 
November  with  rain  they  were  14.5  and  37.1  c.  c.  The  course  of 
daily  evaporation  during  this  week  decreased  from  37.1  c.  c.  for  Mon¬ 
day  to  14.5  c.  c.  for  Wednesday.  There  was  a  slight  increase  on 
Thursday  and  a  drop  on  Friday,  from  which  day  the  daily  amount 
increased  until  the  following  week.  Rain  was  recorded  on  5  days. 
On  Monday  it  was  0.01,  on  Tuesday  it  was  0.95,  on  Wednesday  it  was 
0.26,  on  Thursday  it  was  0.86,  and  on  Friday  it  was  0.08  inch.  The 
day  next  succeeding  the  fall  of  0.95  inch  the  lowest  rate  of  evaporation 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


51 


was  obtained,  and  on  the  day  next  following  the  fall  of  0.86  inch,  the 
next  lowest,  15  c.  c.,  occurred.  After  the  intermediate  amount  of 
rainfall  for  Thursday  the  daily  evaporation  mounted,  until  on  Sunday 
it  was  33.4  c.  c.  During  the  week  the  mean  temperature  decreased 
daily  from  80°  on  Monday  to  59.5°  on  Wednesday,  the  day  following 
the  heaviest  rain,  increasing  slightly  on  Thursday,  following  a  smaller 
rainfall  of  Wednesday,  and  decreasing  Friday  to  53.5°,  following  the 
heavy  rain  of  the  previous  day,  after  which  it  gradually  increased  to 
56°  on  Sunday.  The  daily  variation  in  temperature  and  of  rainfall 
is  thus  reflected  in  the  readings  of  the  atmometer. 

In  table  1 1  are  given  the  normal  and  current  rainfall  for  the  months 
of  September  to  November,  together  with  the  monthly  evaporation 
and  the  P/E. 

Table  11. — Normal  and  current  precipitation-evaporation  ratios  (P/E)  at  Beaufort 
West  for  September-N ovember  1921 ,  together  with  current  and  normal  rainfall. 


Evapora¬ 

tion. 

Normal 

precipita¬ 

tion. 

Normal 

P/E. 

Current 

precipita¬ 

tion. 

Current 

P/E. 

c.  c. 

inches. 

inches. 

Sept . 

1,414 

0.659 

0.00046 

0.26 

0.000183 

Oct . 

1,381 

.660 

.00047 

0.08 

.000056 

Nov . 

1,350 

.700 

.00051 

2.45 

.001074 

The  normal  P/E  at  Beaufort  West  for  the  months  of  spring,  so  far 
as  is  told  by  the  record  of  1921,  is  relatively  low,  but  considerably 
higher  than  at  Matjesfontein  for  the  same  months,  as  would  be  ex¬ 
pected  from  the  greater  evaporation  at  the  latter  place  and  the  some¬ 
what  smaller  rainfall.  The  larger  current  ratio  for  November, 
equaling  that  for  December  at  Matjesfontein,  indicates  that  the 
summer  ratios  may  be  relatively  high.1  Whether  the  amplitude  of 
the  annual  P/E  ratio  at  Beaufort  West  is  as  great  as  that  for  the  other 
station,  may  be  doubted,  but  can  not  be  safely  predicted  in  the  absence 
of  more  complete  evaporation  records. 

SWAKOPMUND. 

At  Swakopmund  the  atmometer  record  at  hand  is  from  July  1  to 
November  11,  1921.  The  atmometer  was  situated  on  the  roof  of  a 
house  which  w^as  about  14  meters  above  the  level  of  the  sea.  It 
was  exposed  to  the  wind  and  to  the  sun. 

The  mean  annual  rainfall  at  Swakopmund  is  0.67  inch.  No  rain 
occurred  during  the  period  of  observation  above  given.  It  seems 
probable  that  the  rate  of  evaporation  at  Swakopmund,  therefore,  is 

1  The  normal  P/E  for  February  at  Beaufort  West  is  0.0022,  but  the  current  ratio  for  1922  was 
0.00089.  See  preceding  footnote. 


52 


FEATURES  OF  THE  VEGETATION  OF  THE 


little  affected  by  the  rainfall.  But,  on  the  other  hand,  the  ocean 
fog,  which  occurs  frequently,  especially  in  the  warm  seasons,  carries 
much  moisture.  An  additional  climatic  feature,  in  addition  to  temper¬ 
ature,  which  modifies  the  humidity  of  the  air,  is  the  east  wind,  which 
may  be  hot  and  therefore  a  drying  wind.  Thus  the  observer  at  Swa- 
kopmund,  in  commenting  on  the  current  weather  conditions,  often 
mentions  the  occurrence  of  the  east  wind,  with  the  remark  that  it  is 
warm  or  hot,  and  he  also  comments  on  the  presence  of  fog.  The  latter 
apparently  may  occur  for  days  together.  The  east  wind  appears  to 
have  been  especially  frequent  in  July  and  in  August,  and  there  was  some 
fog  in  these  months  as  well.  The  most  pronounced  occurrence  of  fog, 
however,  appears  to  have  been  in  October,  especially  in  the  earlier 
part  of  the  month.  In  late  October  the  notation  was  made  that  the 
weather  was  pleasant.  There  was  an  immediate  relation,  as  will 
appear  directly,  between  these  features  of  the  climate  and  the  rate  of 
evaporation  as  revealed  by  the  record  of  the  atmometer. 

The  total  evaporation  for  July  was  1,039  c.  c.,  for  September  it 
was  613  c.  c.,  and  for  October  it  was  619  c.c.  The  August  record  is  not 
complete.  The  greatest  daily  evaporation  was  85.5  c.  c.,  which  was  the 
average  of  the  evaporation  for  4  days,  and  the  least  daily  evaporation 
was  12  c.  c.,  which  was  the  average  for  11  days.  Although  thus  there  is 
a  great  variation  in  the  daily  amount  of  evaporation  at  Swakopmund, 
it  appears,  so  far  as  the  records  on  hand  are  concerned,  that  for  the 
portions  of  the  seasons  studied  the  usual  daily  evaporation  ranges 
between  20  and  30  c.  c.  During  July  the  amount  of  evaporation  was 
relatively  large.  From  July  9  to  July  13,  for  example,  it  was  342  c.  c. 
At  this  time  there  were  warm  days  and  an  easterly  wind.  From 
August  27  to  September  5  the  total  evaporation  was  111  c.  c.  This 
was  a  period  of  foggy  days.  During  the  first  11  days  of  November  the 
total  evaporation  was  278  c.  c.  In  September  and  October,  except  the 
period  just  given,  the  daily  evaporation  was  21.8,  19.3,  21,  25.7,  and 
25.2  c.  c.  daily,  to  give  the  daily  average  for  successive  weeks.  In  every 
instance  where  the  daily  evaporation  was  high  it  was  associated  with  an 
easterly  wind,  and  in  every  instance  when  the  evaporation  was  low,  as 
in  late  July,  early  August,  and  early  October,  there  were  days  of  fog. 
The  rainfall  records  for  Swakopmund  from  1913  to  1920  do  not  note 
any  precipitation  for  the  months  of  July,  August,  and  September, 
but  the  mean  for  October,  for  this  period,  is  0.07  inch.  From  this  it 
will  appear  that  the  precipitation-evaporation  ratio  is  frequently 
exceedingly  low.  For  October  it  was  0.00011. 

PRETORIA  AND  IRENE. 

The  atmometer  records  for  Pretoria  and  Irene  include  two  series  at 
each  station.  At  Pretoria  two  atmometers  were  situated  in  the 
rockery,  at  the  Division  of  Botany.  At  Irene,  9  miles  distant,  one 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


53 


station  was  near  the  river  and  one  on  a  kopje.  The  altitude  of  Pre¬ 
toria  is  4,471  feet  and  that  of  Irene  4,805  feet  above  the  sea. 

The  atmometer  records  at  hand  for  Pretoria  are  from  the  week  end¬ 
ing  September  10  to  the  week  ending  December  31,  1921.  Of  these 
one  only  will  be  referred  to  in  this  place.  The  least  weekly  amount 
of  water  evaporated  for  the  period  was  for  the  week  ending  December 
9;  it  was  95  c.  c.  Between  September  24  and  October  6  the  amount  of 
evaporation  was  590  c.  c.,  or  approximately  350  c.  c.  per  week.  This 
was  probably  the  highest  weekly  rate.  The  average  daily  evaporation 
for  the  time  was  about  50  c.  c. 

The  total  amount  evaporated  for  the  month  of  Octoberwas  1,129  c.  c. 
and  for  November  717  c.  c.  The  normal  rainfall  for  October  at  Pre¬ 
toria  is  2.95  inches,  and  the  amount  for  the  month  in  1921  was  2.77 
inches.  The  normal  and  current  P/E  ratios  are  therefore  0.0026  and 
0.0024.  The  normal  November  rainfall  is  3.89  and  the  amount  for  the 
month  in  1921  was  4.68  inches.  The  normal  and  current  P/E  ratios 
are  therefore  0.0054  and  0.0065. 

At  Irene  the  records  are  from  August  31  to  the  close  of  the  year 
1921.  The  record  of  the  atmometer  placed  near  the  river  is  complete 
for  September,  October,  and  November.  The  least  weekly  evapora¬ 
tion  for  this  station  was  for  the  week  ending  November  26 — 141  c.  c. 
The  highest  rate  was  for  the  week  ending  September  25,  when  the 
evaporation  was  327  c.  c. 

The  total  evaporation  for  September  was  1,095  c.  e.,  for  October 
983  c.  c.,  and  for  November  493  c.  c.  The  annual  rainfall  at  Irene  is 
nearly  the  same  as  at  Pretoria,  so  that  in  the  absence  of  immediately 
available  rainfall  data  for  this  station  it  will  be  assumed  to  be  quite 
as  at  Pretoria,  both  as  regards  the  normal  and  the  current  amounts 
for  the  months  considered.  For  September  the  normal  precipitation 
at  Pretoria  is  0.38  inch.  The  normal  P/E  for  September  at  the  river 
station,  Irene,  is  therefore  0.00034.  The  rainfall  for  September  1921 
was  0.90  inch,  making  the  current  P/E  0.00082.  The  normal  and 
current  P/E  for  October  is  0.0030  and  0.0028,  respectively.  For 
November  the  two  ratios  are  0.0078  and  0.0094. 

The  station  on  the  kopje,  Irene,  had  a  minimum  weekly  evapora¬ 
tion  of  122  c.  c.  for  the  week  ending  September  10,  and  a  weekly 
maximum  of  328  c.  c.  for  the  week  ending  October  16. 

The  total  evaporation  on  the  kopje  for  September  was  965  c.  c., 
for  October  1,094  c.  c.,  and  for  November  564  c.  c.  Employing  the  pre¬ 
cipitation  for  Pretoria  as  before,  the  following  normal  and  current 
ratios  for  the  P/E  are  obtained:  September,  0.00039  and  0.00093; 
October,  0.0026  and  0.0025;  November,  0.0068  and  0.0082. 

It  will  be  seen  that  in  September  the  P/E  ratio  for  the  station  on 
the  kopje  had  a  somewhat  higher  September  ratio  than  the  other 
station,  but  that  in  October  and  November  the  opposite  condition 


54 


FEATURES  OF  THE  VEGETATION  OF  THE 


obtained.  The  low  ratios  for  September  and  the  higher  ratios  in  the 
two  later  months  of  spring  point  to  the  possibly  normal  seasonal 
march  of  the  moisture  relations. 


Table  12. — Normal  and  current  'precipitation-evaporation  ratios  (P/E)  at  Pretoria  and  two 

stations  at  Irene ,  September  to  November  1921. 


Normal  P/E. 

Current  P/E. 

Pretoria. 

Irene. 

Pretoria. 

Irene. 

River. 

Kopje. 

River. 

Kopje. 

September . 

0.00034 

.0030 

.0078 

0.00039 

.0026 

.0068 

0.00082 

.0028 

.0094 

0.00093 

.0025 

.0082 

October . 

November . 

0.0026 

.0054 

0.0024 

.0065 

SUMMARY. 

The  results  of  the  preliminary  studies  on  evaporation,  as  revealed 
by  readings  of  the  atmometer,  are  at  present  too  fragmentary  for 
important  deductions  or  generalizations.  Certain  features  of  interest, 
however,  are  suggested,  which  can  be  mentioned. 

The  precipitation-evaporation  ratio  is  an  index  of  the  comparative 
aridity  of  a  station  which  is  of  value  for  purposes  of  definition.  This 
gives  either  the  normal  or  the  current  index,  accordingly  as  the  normal 
or  the  current  rainfall  is  employed.  With  the  accumulation  of  evap¬ 
oration  data  the  use  of  the  normal  and  current  evaporation  will 
enhance  the  value  of  the  ratios. 

At  none  of  the  stations  are  the  records  of  sufficient  length  to  give 
the  course  of  the  aridity  index  throughout  the  year.  But  at  the 
National  Botanic  Gardens  the  record  is  sufficiently  long  to  give  some 
idea  of  the  extremes  to  be  expected  in  the  region  of  winter  rains.  The 
highest  index  is  in  July,  when  0.0403  was  obtained,  and  the  lowest, 
0.0005,  was  in  January-February.  Thus  the  differences  in  aridity 
as  between  the  winter  and  the  summer  seasons  at  the  station  are  very 
great  indeed.  In  regions  having  most  of  the  rainfall  in  summer  other 
relations  would  be  expected.  Thus,  at  Grahamstown,  the  July  ratio 
was  0.00072  and  the  ratio  for  October  was  0.0038.  At  Irene  the  index 
for  September  was  0.00034  (river  station),  and  in  November  it  was 
0.078.  At  Pietermaritzburg  the  records  are  for  the  warm  season  only 
and  the  indices  are  accordingly  fairly  high,  ranging  from  0.0029  to 
0.0049. 

For  the  two  stations  in  the  Central  Karroo,  the  records  for  Beaufort 
West  are  September-November,  and  at  Matjesfontein  they  are  from 
October  1921  to  February  1922.  At  the  former  station  about  66 
per  cent  of  the  rainfall  is  in  summer  and  at  the  latter  station  about  63 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


55 


per  cent  is  in  winter.  Although  at  both  stations  the  values  are 
extremely  low,  they,  however,  appear  to  reflect  the  annual  distribution 
of  rain  as  a  prominent  controlling  factor.  This  is  particularly  the 
case  at  Matjesfontein.  The  October  ratio  for  this  place  was  0.00036, 
while  that  for  January  was  0.000016. 

The  generalization  can  be  tentatively  made  that  stations  in  regions 
having  winter  rains  are  relatively  more  arid  than  stations  in  regions 
where  the  precipitation  is  in  summer.  Under  such  conditions  dif¬ 
ference  in  temperature  at  the  time  of  the  dry  period  is  probably  the 
most  important  factor  aside  from  the  fact  of  drought.  The  suggestion 
has  some  support  from  data  derived  from  observations  on  evaporation 
from  free  water-surface.  Graphs  showing  the  P/E  for  Kimberley, 
Johannesburg,  and  Cape  Town  are  given  in  figure  8.  The  mean 
monthly  P/E  ratios  for  the  three  stations  in  the  order  given  are  0.26, 
0.44,  and  0.51.  As  compared  to  Cape  Town,  therefore,  the  coefficient 
of  aridity  of  Kimberley  is  1.9  and  of  Johannesburg  is  1.15.  The 
annual  precipitation  at  Johannesburg  is  about  25  per  cent  more  than 
at  Cape  Town,  87  per  cent  of  which  occurs  in  summer. 

GENERAL  FEATURES  OF  THE  VEGETATION,  ESPE¬ 
CIALLY  OF  THE  MORE  ARID  PORTIONS 
OF  SOUTHERN  AFRICA. 

A  visitor  to  southern  Africa  is  struck  with  the  great  variety  of 
herbaceous  plants  and  shrubs,  and  with  the  paucity  or  total  lack  of 
trees.  The  wild-flower  market  on  Adderley  Street,  Cape  Town, 
and  all  the  country  he  sees  in  spring  are  illustrations  of  the  first, 
and  the  environs  of  the  city,  where  the  flowers  are  most  abundant, 
the  mountains,  and  the  plains  generally,  abundantly  illustrate  the 
second  and  third  impressions.  Indeed,  so  far  as  forests  are  concerned, 
the  visitor  is  likely  to  see  none  at  all,  unless  he  makes  special  trips 
along  the  southern  or  eastern  coastal  belts,  or  to  certain  mountains 
or  to  the  northern  tropics.  The  next  strong  impression  is  likely  to  be 
that  a  very  considerable  portion  of  the  country  over  which  he  travels 
either  is  semi-arid  or  arid,  and  that  such  characteristics  of  the  vegeta¬ 
tion  as  he  hurriedly  notes  are  largely  determined  by  this  feature  of 
the  climate,  and  a  somewhat  closer  acquaintance  is  not  likely  to  wholly 
change  these  impressions. 

Hardly  has  the  visitor  gotten  well  away  from  Cape  Town,  either 
by  motor-car  or  by  train,  and  the  familiar  sight  of  Table  Mountain 
with  its  fringe  of  pigmy  forest  and  forest  patches,  when  the  open 
and  sparsely  inhabited  hinterland  is  entered,  with  agricultural  interests 
in  the  lowlands.  Here  there  are  at  present  but  few  native  trees, 
but  occasional  patches  of  shrubs  are  left  undisturbed,  suggesting 
their  prevalence  in  earlier  times.  These  recall  portions  of  California 


56 


FEATURES  OF  THE  VEGETATION  OF  THE 


and  of  the  Mediterranean  region,  as  the  seaward  slopes  of  the  Atlas 
Mountains,  and  indeed  they  are  referred  to  as  Cape  macchia.1 

As  the  distance  from  Cape  Town  becomes  greater,  a  gradual  change 
is  noticed  in  the  vegetation  which  appears  to  indicate  a  smaller  rainfall. 
Low  hills  here  and  there  are  seen  to  carry  few  shrubs,  but  those  are  low, 
often  cushion-form,  apparently  species  of  Mesembryanthemum.  A 
mountain  gorge  (Hex  River  canyon)  is  entered  and  the  stream  followed 
to  a  high  pass.  On  one  side  one  sees  species  of  Aloe,  with  fleshy  leaves, 
very  like  species  of  Agave  of  America,  and  Cotyledon  with  short,  stout 
stems,  dwarf  trees  with  fleshy  stem  and  branches.  On  the  opposite 
side  of  the  gorge  are  plants  characteristic  of  the  Cape  region.  There 
shrubs  and  herbaceous  plants  are  abundant. 

Crossing  the  pass,  the  course  is  set  in  an  easterly  direction  and 
the  Central  or  Great  Karroo  is  entered.  It  is  not  long  before  the  pass¬ 
ing  vegetation  becomes  more  scattering  and  gradually  changed  from 
that  predominantly  sclerophyll  to  that  predominantly  succulent. 
The  succulents  are  fairly  small.  Trees  are  confined  to  the  water¬ 
courses.  If  the  season  is  late  winter,  or  spring,  the  plains  and  kopjes 
are  brilliant  with  the  wealth  of  flowers  with  which  the  herbaceous  plants 
and  low  shrubs,  especially  species  of  Mesembryanthemum,  are  covered, 
and  the  Karroo  has  become  a  garden  whose  like  is  doubtfully  to  be 
found  elsewhere. 

Thus  far  the  visitor  has  been  passing  through  a  region  in  which  the 
rains  are  chiefly  in  the  cool  seasons  to  one  where  the  summer  rains 
begin  to  be  felt,  and  finally,  as  the  train  climbs  the  escarpment  in  the 
northeasterly  side  of  the  Central  Karroo,  the  region  dominated  by  the 
rains  of  summer  is  entered.  Here  the  vegetation  again  suffers  a 
noticeable  change.  The  succulents  characteristic  of  the  Karroo,  and 
indeed  sclerphyllous  shrubs  as  well,  are  less  abundant  or  largely 
wanting.  Grass  becomes  the  leading  characteristic  of  the  vegetation, 
at  least  of  the  plains.  The  country  is  apparently  not  so  arid  as  the 
Karroo  of  lower  altitude,  farther  to  the  south. 

Passing  across  the  northern  portion  of  the  Upper  Karroo,  and  es¬ 
pecially  into  and  through  Greater  Namaqualand,  more  arid  conditions 
are  again  largely  encountered.  Grass  becomes  a  less  important 
characteristic  of  the  vegetation,  shrubs  are  low,  and  scattering  sclero- 
phylls  and  trees  are  apparently  confined  to  the  savannah  forest  west¬ 
ward  of  the  mountains,  or  to  the  most  prominent  water-courses. 

Across  the  semi-arid  plain  (bajada)  west  of  the  mountains  of  the 
Protectorate  of  Southwest  Africa,  grass  is  again  an  important  feature 
of  the  vegetation,  but  in  places  terete-stemmed  euphorbias  dominate. 
As  the  main  mountains  are  left  behind,  on  the  way  to  Swakopmund, 
the  vegetation  becomes  increasingly  sparse  and  the  region  increasingly 
arid,  until  within  50  miles,  more  or  less,  of  the  coast  true  desert  con- 


1  A  guide  to  botanical  survey  work:  Mem.  No.  4,  Botanical  Survey  of  South  Africa,  p.  59,  1922. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


57 


ditions  are  encountered.  This  is  the  Namib,  one  of  the  most  arid 
regions  known,  with  a  rainfall  on  the  coast  of  1  inch  or  less  annually. 
Seasonal  changes  make  here  relatively  little  difference  in  the  general 
appearance  of  such  a  country,  where  shrubs  are  small  and  are  con¬ 
fined  to  drainage  channels,  where  trees  are  along  the  main  streams  only, 
and  where  the  ground  is  for  the  most  part  quite  bare.  Nevertheless, 
plants  are  to  be  found  here  of  great  interest  and  uniqueness,  as  Wei- 
witschia  and  Acanthosicyos,  although  neither  can  be  seen  from  the 
railway  coach. 

Retracing  his  course  to  De  Aar,  the  visitor  is  again  impressed  by  the 
vast  treeless  plains,  the  southern  extension  of  the  Kalahari,  with 
bunch-grass  and  shrubs,  mainly  on  the  kopjes.  Trees  are  confined  to 
the  immediate  vicinity  of  the  rare  water-courses,  as  the  Orange  River. 
Northward  from  De  Aar  the  High  Veld  is  crossed,  a  region  of  grassy 
plains,  and  kopjes  with  shrubs  and  trees  along  the  drainage  channels. 
In  places,  however,  where  the  plains  are  better  watered,  there  appears 
a  sparse  covering  of  sclerophylls.  The  Zoutpansberg  mountains  are 
passed  through  at  length,  and  here  appear  for  the  first  time  tree-like 
euphorbias,  with  fairly  small  trunks  but  with  heavy  crowns.  North 
of  the  mountains  the  Low  Veld  is  entered,  with  the  open  forest  of  low 
trees  and  high  shrubs,  and  with  scattering  baobab  ( Adansonia  digi- 
tata),  which  is  the  largest  or  at  least  the  most  massive  tree  of  southern 
Africa.  The  presence  of  occasional  groups  of  palms  ( Phoenix  sp.)  and 
the  cultivation  of  subtropical  fruits  (at  Messina)  point  to  the  sub¬ 
tropical  or  tropical  character  of  the  northernmost  portion  of  the  Union. 

The  High  Veld  and  the  Low  Veld,  as  pointed  out  above,  are  served 
by  rains  in  summer.  The  winters  are  dry  and  may  be  fairly  cold. 
These  environic  characteristics  are  reflected  in  the  abundance  of 
grass  and  the  presence  of  deciduous  perennials  as  a  marked  floral 
feature  as  opposed  to  the  almost  exclusive  development  of  evergreen 
species  in  the  extreme  southwest. 

Finally,  the  visitor  is  impressed  on  every  side  with  the  evident 
close  relation  between  the  character  of  the  vegetation,  including  often 
the  local  distribution,  and  the  general  facts  as  to  the  amount,  and 
seasonal  distribution  of  the  rainfall.  And  an  important  feature  of 
the  environmental  complex  is  the  fact  of  often  long  yearly  periods 
of  drought,  which  may  be  associated  with  seasons  of  high  temper¬ 
ature,  in  which  event  the  aridity  is  particularly  intense. 

To  the  Little  Karroo  via  Mossel  Bay  and  George  and  through 
the  Outeniqua  Mountains  from  the  Cape  Peninsula  gives  the  visitor 
interesting  glimpses  of  the  south  coast.  The  road  leads  through 
large  plantations  of  trees,  including  the  Pinus  insignis  of  California, 
which  have  the  appearance  of  being  unusually  vigorous  and  free  from 
diseases.  An  open  farming  country,  rolling,  and  not  far  from  the  shore 
at  any  time,  is  crossed,  and  here  and  there  the  way  nears  the  coastal 


58 


FEATURES  OF  THE  VEGETATION  OF  THE 


range  of  mountains,  where  sclerophyllous  shrubs  and  occasional  leafy 
succulents  are  met,  and  in  the  washes  a  few  native  trees.  At  George, 
owing  to  the  large  rainfall,  which  is  equally  distributed  month  by 
month  through  the  year,  the  countryside  recalls  portions  of  England, 
with  green  fields  and  trees  and  shrubs  along  the  streets,  and  has  little 
in  common  with  southern  Africa  as  the  casual  traveler  sees  it.  The 
roads  through  the  mountains  to  Oudshoorn  give  at  first  glimpses  of 
sclerophylls  and  later,  as  the  northern  slopes  of  the  mountains  lead 
to  the  Little  Karroo,  succulents  of  numerous  kinds,  many  of  large 
size,  are  to  be  seen.  Trees  follow  the  water-courses,  and  there  is 
much  cultivation,  including  fields  of  alfalfa  as  food  for  ostriches,  the 
raising  of  which  is  here  an  important  industry. 

BOTANICAL  REGIONS  OF  SOUTHERN  AFRICA. 

Turning  from  such  general  considerations,  some  of  the  leading 
characteristics  of  the  vegetation  of  the  subcontinent  may  be  pre¬ 
sented  somewhat  more  exactly.  Thus  the  leading  botanical  prov¬ 
inces  as  recognized  by  the  Botanical  Survey  of  the  Union  are  outlined 
in  figure  7.  They  have  been  characterized  by  Pole  Evans  as  follows:1 

“That  part  of  Africa  which  lies  South  of  latitude  22°  falls  naturally  into 
two  main  botanical  regions — the  Cape  Region  and  the  South  African  region. 
The  former  is  of  comparatively  small  extent  and  occupies  the  angular  strip 
of  country  in  the  southwest  portion  of  the  Province  of  Good  Hope.  The 
latter  is  of  vast  proportions  and  covers  the  rest  of  the  country  under  review. 
The  vegetation  of  the  Cape  region  has  a  remarkable  uniformity  of  character, 
while  that  of  the  South  African  region  exhibits  a  great  variety  of  types  ranging 
from  typical  desert  to  tropical  forest.  The  Cape  region  differs  from  the  South 
African  region  in  one  important  climatic  factor.  It  experiences  a  winter 
rainfall,  whereas  summer  rains  fall  over  the  greater  part  of  the  South  African 
region.  This  difference  in  the  seasonal  distribution  of  the  rainfall  is  mainly 
responsible  for  the  marked  dissimilarity  of  the  vegetation  of  the  two  regions. 
So  different  are  they  in  aspect  and  composition  that  they  have  little  in  common 
and  might  well  belong  to  two  widely  separated  countries.” 

There  is  a  striking  resemblance  between  the  general  aspect  of  the 
flora  of  the  Cape  region  and  that  of  the  Mediterranean  region,  as 
well  as  that  of  portions  of  California.  Pigmy  forests  are  the  rule, 
and  there  is  a  great  variety  of  bulbous  and  tuberous  plants.  Species 
characteristic  of  the  Cape  “macchia,”  or  chapparal,  are  apparently 
projected  along  favorable  lines  of  migration  into  the  other  botanical 
regions,  where  there  is  more  or  less  mingling  in  a  transition  zone,  as 
along  the  borders  of  the  Karroos. 

It  is  the  South  African  region,  however,  with  which  this  study 
is  mainly  concerned.  This  may  be  divided,  according  to  the  author 

1  The  main  botanical  regions  of  South  Africa,  in  A  guide  to  botanical  work.  Bot.  Survey  of 
South  Africa,  Mem.  No.  4,  1922,  p.  49. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


59 


quoted  above,  into  the  following  botanical  provinces:  the  Nama- 
qualand  Desert  province;  the  Karroo  province;  the  Kalahari  park  and 
bush  province;  and  the  South  African  steppe  and  forest  province. 
The  extent  and  situation  of  the  four  provinces  are  indicated  in  the 
figure. 

The  first  three  botanical  provinces  range  from  desert  to  semi-arid 
and  include  a  great  variety  of  plant  forms,  mainly,  however,  with 
marked  xerophytic  characters.  Nevertheless,  there  are  well-marked 
differences,  as  the  following  characterizations,  taken  from  several 
sources,  including  publications  of  the  Botanical  Survey,  will  indicate. 

In  the  Namaqualand  province  the  “general  aspect  of  the  vege¬ 
tation  is  that  of  widely  separated  xerophytic  shrubs  and  bushes,  with 
a  fair  proportion  of  succulent  plants  in  the  low-lying  valleys  and  on 
the  rocky  outcrops.  Grasses  do  not  form  a  conspicuous  feature  of  the 
vegetation,  but  they  occur  on  the  high  plateaus  and  sandy  plains, 
where  their  vegetative  period  is  short  and  where  they  are  always  of 
a  tufted  habit.”  In  this  province  are  to  be  found,  among  other 
species,  Aloe  dichotoma,  Euphorbia  virosa,  E.  dinteri,  Sterculia  gurichii , 


Fig.  7. — Main  botanical  regions,  after  Pole  Evans.  I.  Karroo  province  with  position  and 
extent  of  Central  or  Great  Karroo  indicated  in  southern  portion,  with  upper  Karroo 
nor  of  the  escarpment.  II.  Namaqualand  desert  province.  III.  Cape  region.  IV. 
Kalahari  park  and  bush  province.  V.  High  veld,  in  western  portion,  steppe  and  forest 
province  in  mountains,  and  eastern  grass  veld  and  coast  forest  area  to  the  east. 


60 


FEATURES  OF  THE  VEGETATION  OF  THE 


Cissus  crameriana,  and  Pachypodium  giganteum  on  the  hills;  Parkin - 
sonia  africana,  Acacia  hebeclada ,  A.  tenax,  A.  hereroensis,  Boscia 
fcetida,  Rhigozum  trichotomum,  Catophractes  alexandri ,  Sarcocaulon 
burmanni,  S.  rigidum ,  and  Hoodia  gordoni  on  the  plains;  Euphorbia 
gregaria  in  the  low-lying  valleys,  and  along  the  dry  river-beds  Acacia 
giraffce,  A.  albida,  Combretum  primigenum ,  Euclea  pseudebenum , 
Tamarix  articulata,  and  Sisyndite  spartea,  to  quote  from  Pole  Evans.1 

In  the  desertic  Namib  the  same  writer  gives  the  following  as  among 
the  more  typical  species:  Acanthosicyos  horrida,  Arthrcerua  leub- 
nitzice,  Euphorbia  branchiata,  Mesembryanthemum  marlothii,  Welwit- 
schia  mirabilis,  and  Zygophyllum  stapfii. 

The  Karroo  province  is  divided  into  the  Upper  Karroo,  or  plains, 
lying  between  3,000  and  6,000  feet  above  the  sea,  and  the  Karroo, 
which  has  an  altitude  of  1,000  to  3,000  feet.  Pole  Evans  states: 

“[The]  country  generally,  except  after  recent  rains,  has  a  semi-desert  appear¬ 
ance.  The  vegetation  is  composed  largely  of  xerophytic  shrubs,  shrublets, 
and  succulents.  Trees  are  almost  entirely  absent  except  along  the  river-beds. 
Some  of  the  more  typical  plants  are:  Galenia  africana,  Melianthus  comosus, 
Salsola  aphylla,  Euphorbia  mauritanica ,  Mesembryanthemum  spinosum, 
Aitonia  capensis,  Sutherlandia  frutescens,  Lycium  afrum,  L.  austrinum ,  Chryso- 
coma  tenuifolia,  Pentzia  incana,  Othonna  pallens,  Arctotis  stoechandifolia,  and 
Gnidia  polycephala.” 

In  the  Upper  Karroo  small  shrubs  belonging  to  the  Compositse  are 
the  dominating  type,  but  in  the  Karroo  proper  succulents,  usually 
small,  dominate.  Among  these  are  species  of  Aloe,  Cotyledon ,  Gasteria, 
Haworthia,  Mesembryanthemum,  and  Pelargonium. 

The  Kalahari  park  and  bush  province,  according  to  the  same 
author,  occupies  the  vast  tract  of  country  which  forms  the  central 
portion  of  South  Africa.  This  province  extends  into  the  Tropics. 
The  vegetation  is  composed  of  trees,  scattered  bush,  and  grass.  Over 
the  greater  portion  of  the  country  the  general  aspect  of  the  vegetation 
is  park-like.  Toward  the  east,  where  the  rainfall  is  higher  and  more 
regular,  the  bush  becomes  denser  and  thicker.  Thorn  trees,  mostly 
species  of  acacia,  are  the  dominating  feature  of  the  bush.  Among  the 
widely  distributed  genera  which  occur  in  the  province  are  Acacia, 
Boscia,  Dichrostachys,  Grewia,  Olea,  Peltophorum,  Rhus,  Royena, 
Tarchonanthus,  and  Zizyphus.  In  the  northern,  tropical  portion  there 
are  among  other  species  Adansonia  digitata,  Copaifera  mopane,  and 
Sterculia  spp.  Of  these  the  striking  baobab  tree  (. Adansonia )  has 
already  been  mentioned.  (Plate  5.) 

The  South  African  steppe  and  forest  province  occupies  a  large  por¬ 
tion  of  the  eastern  side  of  South  Africa,  according  to  Pole  Evans. 
“In  marked  contrast  to  the  areas  already  dealt  with,  this  tract  of 
country  is  well  covered  with  grass;  in  fact,  grass  is  dominant  every- 


1  Bot.  Sur.  So.  Africa,  Mem.  No.  4,  1922,  p.  51. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


61 


where  except  where  forest  patches  occur.”  The  topography  is  ex¬ 
tremely  varied,  ranging  from  the  sea-strand  to  the  most  important 
mountains  of  South  Africa,  and  beyond  to  the  High  Veld,  or  steppe, 
with  characteristic  savannah-forest  and  steppe  vegetation,  consisting 
of  large  numbers  of  species,  many  of  which  have  tropical  affinities.  It 
is  in  the  eastern  savannah  that  the  large  species  of  Euphorbia  are  to 
be  found.  This  province  has  features  of  great  botanical  interest, 
certain  of  which  will  be  referred  to  in  another  place  and  connection. 
On  the  steppe  the  dominant  grass  is  Themeda  triandra.  The  trees 
of  the  Drakensberg  forest  are  made  up  largely  of  Podocarpus  elongata , 
P.  latifolia,  Olea  laurifolia,  Celtis  kraussiana,  Curtisia  faginea ,  and 
Xymalos  monospora.  In  the  savannah  east  of  the  mountains  are 
Acacia  karoo  and  others  of  this  genus,  Dichrostachys  nutans ,  Ehretia 
hottentotica,  and  Zizyphus  mucronata.  The  chief  succulents  are 
Euphorbia  grandidens,  E.  cooperi,  E.  ingens ,  Aloe  ferox,  A.  bainesii ,  and 
A.  marlothii.  The  coastal  forest  includes  many  trees  most  of  which 
have  tropical  affinities,  and  among  the  genera  are  Rhus ,  Albizzia, 
Millettia,  Harpephyllum,  Macaranga,  Rauwolfia,  Hyphema,  Phoenix, 
and  Strelitzia. 


62 


FEATURES  OF  THE  VEGETATION  OF  THE 


VEGETATION  IN  PORTIONS  OF  NAMIB  DESERT 
AND  OF  CENTRAL  KARROO. 

Although,  as  mentioned  earlier,  the  writer  had  an  opportunity  to 
see  something  of  the  flora  of  the  Cape  region  of  the  Little  Karroo,  as 
well  as  of  the  High  Veld  and  Low  Veld,  and  of  the  Eastern  grass  veld 
between  Pietermaritzburg  and  the  sea,  most  of  his  attention  was  given 
to  two  stations  in  the  Central  Karroo,  with  a  short  visit  to  Southwest 
Africa  and  the  Namib.  The  following  pages,  therefore,  have  to  do 
mainly  with  the  Karroo  and  the  Namib,  the  former  at  and  about 
Beaufort  West,  Prince  Albert  Road,  and  Matjesfontein,  and  the 
latter  at  Swakopmund  and  the  region  east  for  about  50  km.,  including 
the  habitat  of  Welwitschia. 

At  the  stations  mainly  studied  observations  were  made  on  the 
most  striking  perennials,  and  numerous  tests  were  made  on  the  trans¬ 
piring  power  of  several.  Local  distributional  characteristics  are  sum¬ 
marized  in  connection  with  the  brief  discussion  next  to  follow,  in 
which  some  account  is  given  of  root  characters.  Salient  anatomical 
and  morphological  features,  with  their  possible  significance,  follow  the 
last,  and  finally  the  results  of  the  work  on  the  transpiring  power 
is  presented. 

The  plates  to  be  found  at  the  end  of  the  general  study  are  designed 
to  illustrate  the  character  of  the  vegetation  and  of  the  habitat  in  a 
manner  and  to  a  degree  not  possible  by  other  means. 

THE  NAMIB. 

The  Namib  was  crossed  on  the  way  from  Windhoek  to  Swakopmund. 

In  the  relatively  short  distance,  about  100  miles,  which  separates 
the  mountains  of  Southwest  Africa  from  the  coast,  an  increase  in 
aridity  is  experienced  from  semi-arid  at  the  base  of  the  mountains  to 
desert  along  the  sea.  There  is  a  corresponding  marked  change  in  the 
vegetation. 

In  the  vicinity  of  IJsakas,  altitude  2,865  feet  above  the  level  of  the 
sea,  are  good-sized  trees,  largely  species  of  Acacia,  on  the  flats,  with 
park-like  openings  where  grass  occurs.  On  the  lower  slopes  of  the  hills 
are  shrubs  and  grass.  To  the  west  of  Usakas,  and  for  about  50  miles, 
is  the  transition  between  the  Acacia  park  forest  and  the  Namib  Desert.1 

Near  Aukas,  altitude  2,981  feet,  grass  is  abundant  and  there  are 
clumps  of  a  terete-stemmed  Euphorbia  forming  massive  clumps  from 
2  to  3  meters  high  and  possibly  5  meters  in  diameter.  In  places  these 
are  so  abundant  as  to  constitute  bush  formation.  A  few  miles  west 
of  Aukas  the  line  swings  out  from  the  hills  and  enters  a  wide  plain 
on  which  the  Euphorbia  is  especially  abundant,  appearing  to  be  the 
only  shrubby  species.  But  there  are  scattering  grasses. 

1  Some  notes  on  a  journey  from  Walfish  Bay  to  Windhuk.  H.  H.  W.  Pearson.  Bull.  Misc. 
Information,  No.  9,  1907,  Roy  Bot.  Gard.  Kew. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


63 


At  Ebony,  at  a  somewhat  greater  altitude,  the  rolling  plains  are 
well  covered  with  grass.  There  are  scattering  and  small  sclerophyl- 
lous  shrubs  but  no  euphorbias.  This  condition  persists  for  a  few  miles, 
when  with  continuous  and  rapid  decrease  in  altitude  the  aridity 
quickly  becomes  more  intense  and  the  Namib  Desert  is  reached. 

The  flora  of  the  Namib,  in  the  vicinity  of  Swakopmund,  is  more 
diverse  than  the  extremely  small  rainfall  of  the  region  would  lead  one 
to  suppose.  The  plains  are,  it  is  true,  fairly  devoid  of  vegetation,  bub 
there  are  shrubs  and  even  trees  of  some  size  along  the  bed  of  the  Swakop 
River.  Pearson  states: 

“  On  the  rocky  sides  and  sandy  floors  of  the  lateral  valleys  of  the  Swakop  and 
the  Khan  are  found  Welwitschia,  Commiphora,  ....  Aristidce,  and  other 
grasses,  with  Aloe  dichotoma ,  Linn,  f.,  Sarcostemma  viminale  R.  Br.,  a  cacoid 
Euphorbia  with  stems  6  to  8  feet  high  and  a  Hoodia  (?  H.  Gordoni  Sweet)  of 
similar  habit.  These  and  other  Namib  forms  here  meet  and  mingle  with 
plants  belonging  to  the  entirely  different  flora  which  prevails  in  the  beds  of  the 
main  rivers  and  consists  of  species  which  are  at  home  on  higher  levels  to  the 
north  and  east  where  these  rivers  take  their  rise.  The  dry  sandy  bed  of  the 
Swakop  at  Haikamchab  (750  ft.  alt.)  supports  a  flora  rich  in  individuals  and 
fairly  so  in  species,  which  includes  among  its  more  predominant  forms 
Acacia  albida  Del.,  the  ‘Ana’  tree;  A.  giraffce  Wild.,  the  “  Camel  thorn,” 
perhaps  the  commonest  tree  in  Damaraland;  Ficus  demarensis  Engl.;  Euclea 
pseudebenus  E.  Meyer;  Tamarix  articulata  Vahl.;  the  Walfish  Bay  Caroxylon, 
here  a  shrub  15  feet  high,  with  an  undergrowth  of  the  sand-binding  thorny 
grass,  Eragrostis  spinosa  Trim,  which  covers  large  areas  of  the  river-bed 
almost  to  the  exclusion  of  other  plants  of  low  habit;  a  Tribulus  (?  T.  erectus 
Engl.)  1  to  3  ft.  high,  with  handsome  yellow  flowers;  a  white-flowered  Helio- 
tropium  (?  H.  albiflorum  Engl.);  and  a  white  Cleome.  The  margins  of  the 
sandy  bed  are  in  some  places  fringed  by  a  dense  scrub  of  reeds/ ’ 

The  foregoing  description  of  the  vegetation  of  the  main  river-beds 
and  lateral  valleys  applies  to  a  region  about  25  miles  east  of  Swakop¬ 
mund.  This  region,  as  stated  above,  is  one  of  the  habitats  of  Wel¬ 
witschia  mirabilis.  According  to  Pearson,  the  area  where  this  rare 
species  occurs  is  about  25  miles  north  and  south  and  about  15  miles 
east  and  west,  but,  also  according  to  Pearson,1  the  species  occurs 
in  a  second  locality  about  400  miles  to  the  north.  The  eastern  limit 
of  its  distribution  appears  to  coincide  with  eastern  limit  of  the  penetra¬ 
tion  of  the  ocean  fog,  and  the  altitude  seems  to  be  300  meters  more  or 
less.  Where  I  saw  the  species  was  in  a  shallow  wash,  possibly  250 
meters  wide,  which  slopes  gradually  to  the  Swakop  River,  about  2 
miles  to  the  north.  It  was  on  the  edge  of  a  plateau.  To  the  south 
was  a  rim  of  somewhat  higher  ground  from  which  other  shallow  and 
smaller  drainage  channels  made  their  way.  The  ground  was  quite 
bare  of  vegetation,  except  in  the  channels,  where  was  seen  a  straggling 
line  of  scattered  small  shrubs.  Of  these  the  most  conspicuous  at  the 

1  Some  observations  on  Welwitschia  mirabilis  Hooker.  H.  H.  W.  Pearson.  Phil.  Trans.  Roy. 
Soc.  Lond.,  Ser.  B,  vol.  198,  p.  269,  1906. 


64 


FEATURES  OF  THE  VEGETATION  OF  THE 


time,  June  28,  was  Zygophyllum  stapfii,  with  its  compact  growth  habit 
and  dark-green  round  and  fleshy  leaves  (plates  2c,  3c).  There  also 
was  Asclepias  filiformis,  with  its  wand-like  shoots,  and  Bauhinia 
marlothii ,  with  fairly  large  leaves,  and  Arthrcerua  leubnitziea,  with 
rather  fleshy  stems  (plate  3a,  3b).  Five  specimens  of  Welwitschia 
were  seen  widely  separated  on  the  plain  (plates  1  and  2).  Of  these, 
two  were  about  2  meters  from  tip  to  tip  and  the  balance  were  smaller. 
It  will  be  seen  from  the  figures  that  the  habitat  of  the  species  is  ex¬ 
tremely  arid,  although  the  presence  of  any  vegetation  indicates  a 
certain  amount  of  available  water.  In  fact,  the  moisture  relations 
are  possibly  better  than  nearer  the  coast,  owing  in  part  to  the  higher 
elevation.  The  occurrence  of  perennials  along  the  small  drainage- 
channels  on  the  plain  and  in  the  bed  of  the  Swakop  River  near  the 
habitat  of  Welwitschia ,  suggests  the  presence  of  subterranean  water. 
The  heavy  ocean  fog,  also,  which  penetrates  as  far  as  this  place, 
probably  aids  directly  as  well  as  indirectly  to  alleviate  the  conditions 
of  aridity.  That  Welwitschia ,  as  well  as  Bauhinia ,  growing  near  by 
are  in  position  to  obtain  water  is  evidenced  by  the  fact  that  they  give 
off  water  in  transpiration,  as  mentioned  in  another  place.  In  fact, 
the  transpiring  power  of  both  of  these  plants  on  June  28  was  found  to 
be  relatively  high,  which  suggested  that  the  species  may  not  be  so 
markedly  xerophytic  as  the  appearance  of  the  habitat  would  appear 
to  require. 

Between  the  habitat  of  Welwitschia  and  Swakopmund  the  wagon- 
road  goes  for  the  most  of  the  way  on  the  south  side  of  the  river,  but 
crosses  it  before  reaching  that  town.  The  vegetation  on  the  plateau 
and  along  the  track  is  as  just  sketched.  Here,  in  shallow  drainage 
depressions,  were  seen  Arthrcerua  leubnitzice,  with  crowded  fleshy 
branches,  Celosia  spathulifolia,  Senecio  sp.,  and  Zygophyllum  stapfii. 

Crossing  the  river-bed  some  distance  to  the  east  of  Swakopmund,  one 
sees  specimens  of  Acanthosycios  horrida  (plate  1),  which  forms  clumps 
about  which  the  sand  congregates,  forming  hillocks.  The  species 
is  leafless,  but  richly  branched,  and  bears  countless  short,  stout  spines 
which  are  the  homologues,  according  to  Pearson,  of  tendrils.  It  is 
more  xerophytic  in  appearance  than  in  reality.  It  has  long  roots 
which  tap  perennial  water-supply,  and  Pearson  states  that  watery  fluid 
exudes  in  drops  from  the  cut  ends  of  the  assimilating  stems.  Although 
I  searched  several  shrubs,  I  found  but  one  fruit.  The  melons  of  this, 
the  naras,  are  in  great  demand  among  the  native  blacks  and  constitute 
an  important  article  of  food,  being  eaten  fresh  in  summer  and  dried 
for  subsequent  use  in  winter.1 

1  The  Namib  was  visited  by  the  writer  in  June-July,  1921,  when  the  fairly  abundant  flora  of 
the  Swakop  Valley  was  seen.  In  March  1923,  according  to  a  correspondent,  and  as  a  result  of 
unusually  heavy  rains  in  the  mountains,  the  valley  of  the  river  was  flooded,  with  the  effect  that 
“there  is  no  trace  of  vegetation  left  in  its  bed,  nor  any  tree  on  its  banks  for  miles.”  From  this 
it  would  appear  possible  that  the  “naras”  may  have  been  washed  into  the  sea. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


65 


Species  of  Tamar  ix  and  Nicotiana ,  as  well  as  of  halophytic  grasses, 
are  common  in  the  river-bed  at  Swakopmund,  and  the  first  named 
occurs  occasionally  among  the  sand  dunes  which  lie  along  the  south 
side  of  the  river.  On  the  plain  back  of  the  village  there  appears  to  be 
little  or  no  vegetation  of  any  kind. 

THE  CENTRAL  KARROO. 

Beaufort  West. 

At  Beaufort  West,  in  the  northeasterly  part  of  the  Central  Karroo, 
the  vegetation  of  the  dolorite  kopjes  near  town  was  examined  and  some 
visits  were  paid  to  the  wide  mouth  of  a  canyon  about  6  miles  east. 
The  kopjes  run  in  general  northwest-southeast  direction,  so  that  there 
are  northern  and  southern  aspects. 

In  general  it  can  be  said  that  the  veld  around  the  kopjes  and  the 
kopjes  as  well  have  a  vegetation  which  is  mainly  composed  of  sclero- 
phyllous  shrubs  and  shrublets,  which  on  the  plain  are  scattering,  and 
on  the  kopjes  are  more  numerous  on  the  southern  than  on  the  northern 
side,  but  nowhere  form  a  thicket  (plate  8). 

Two  quadrats,  10  by  10  meters  in  size,  and  situated  on  the  northerly 
and  southerly  faces  about  100  meters  apart,  were  studied  somewhat 
intensively.  The  results  give  a  fair  picture  of  the  character  of  the 
vegetation  of  the  kopje,  as  well  as  the  reaction  of  the  vegetation  to  the 
two  different  aspects. 

In  the  quadrat  on  the  south  face,  76  perennials  were  counted,  of 
which  there  w~ere  Lycium  sp.,  Carissa  ferox,  Grewia  cana ,  Euphorbia 
mauritanica,  Cotyledon  wallichii,  and  Mesembryanthemum  sp.  (plates 
7a,  8c).  One  of  the  characteristics  of  the  quadrat  was  the  occurrence 
of  several  different  species  in  a  single  aggregate.  Thus,  for  example, 
plate  9  shows  Gasteria  disticha  growing  at  the  base  of  Euphorbia 
mauritanica  and  Lycium,  and  in  plate  10  Crassula  quadrangularis  is 
shown  at  the  base  of  Lycium  sp. 

In  the  quadrat  on  the  northern  exposure,  45  perennials  were  found, 
among  which  were  Euphorbia  mauritanica,  Pentzia  virgata,  and  Aspar¬ 
agus  striatus.  Thus  perennials  of  whatever  kind  are  less  abundant  on 
the  northern  slope,  and  also  the  composition  is  unlike  on  the  two 
aspects.  On  the  northern  exposure  occur  Aloe  schlechteri  and  Euphor¬ 
bia  stellcespina,  neither  of  which  are  on  the  opposite  slope  (plate  8a, 
8b).  On  the  other  hand,  such  species  as  Cotyledon  wallichii  and 
Crassula  quadrangularis  of  the  southern  aspect  are  not  to  be  found  on 
the  northern  side.  Thus  aspect  “ preference”  is  clearly  indicated. 

An  attempt  was  made  to  determine  possible  differences  in  the 
two  aspects  as  regards  the  evaporation  power  of  the  air,  but  without 
satisfactory  success,  owing  to  the  removing  of  one  of  the  atmometers 
with  reservoir  by  some  unknown  person.  However,  the  following  was 
learned :  For  the  week  ending  August  22  the  atmometer  on  the  upper 


66 


FEATURES  OF  THE  VEGETATION  OF  THE 


northern  slope  lost  562  c.  c.  of  water,  and  during  the  same  week  the 
atmometer  in  Beaufort  West,  about  1.5  miles  distant,  had  a  water-loss 
of  314  c.  c.  On  the  following  week  the  one  on  the  southern  slope 
lost  356  c.  c.  while  the  one  in  town  showed  271  c.  c.  of  water  evaporated. 
These  results  indicate  a  probable  greater  dryness  of  the  air  on  the 
northern  than  on  the  southern  slope  of  the  kopje,  as  well  as  the  greater 
dryness  of  the  stations  on  the  veld  as  compared  to  the  one  in  town. 

Among  other  noteworthy  species  which  occur  on  the  upper  slopes 
of  the  kopje  is  Gymnosporia  buxifolia  (?)  which  has  the  distinction  of 
possessing  spines  of  unusual  size  (plate  7b,  7c).  These  are  apparently 
dwarf  branches,  inasmuch  as  at  certain  stages  in  development  they  bear 
leaves.  One  spine  was  found  which  was  17  cm.  in  length. 

The  following  species,  in  addition  to  several  which  were  not  known, 
were  found  on  the  kopje:  Aloe  schlechteri ,  Aptosimum  indivisum , 
Asparagus  stipulaceus ,  A .  striatus,  Aster  filifolius,  Carissa  ferox,  Coty¬ 
ledon  decussata,  C.  wallichiiy  Crassula  quadr angular is ,  Euphorbia 
mauritanica,  E.  stellcespina,  Gasteria  disticha,  Gazania  pinnata,  Gna- 
phalium  sp.,  Grewia  cana,  Gy?nnosporia  buxifolia  (?),  Hermannia 
spinosa ,  Indigofera  sp.,  Kleinia  articulata ,  K.  radicans,  Lycium  sp., 
Massonia  latifolia,  Mesembryanthemum  densum,  M.  spinosum,  Nemesia 
sp.,  Osteospermum  sp.,  Pentzia  virgata ,  Royena  pollens ,  Senecio  longi - 
folius,  Stapelia  sp.,  Ursinia  sp.,  and  Viscum  rotundifolium  on  Gym- 
no  sporium  buxifolium. 

The  dolorite  kopjes  of  which  the  vegetation  was  sketched  in  the 
preceeding  paragraphs  run  nearly  at  right  angles  to  Nieuweveld 
Mountains,  immediately  on  the  north,  which  separate  this  portion 
of  the  Central  Karroo  from  the  higher  Upper  Karroo.  There  does 
not  appear  at  this  place  to  be  direct  connection  between  the  mountains 
and  the  kopjes  along  which  plants  of  the  two  divisions  of  the  Karroo 
can  readily  migrate.  Accordingly  advantage  was  taken  of  visiting  the 
mouth  of  a  canyon  opening  toward  the  southwest  from  the  Nieuweveld 
Range  and  distant  from  Beaufort  West  about  6  miles.  The  mouth  of 
the  canyon  is  flanked  on  either  side  by  flat-topped  projections  from  the 
main  range.  There  are  long  talus  slopes  ending  abruptly  at  pro¬ 
nounced  cliffs.  From  the  stream  to  the  mesas  above  is  possibly  a 
vertical  distance  of  1,000  feet.  Although  the  kopjes  nearer  Beaufort 
West  are  overrun  by  sheep  and  goats,  so  that  the  vegetation  at  present 
to  be  found  there  are  species  of  no  economic  importance,  this  is  appar¬ 
ently  not  so  much  the  case  at  the  canyon  referred  to.  Even  here, 
however,  there  are  not  only  wandering  flocks,  but  a  natural  fauna  that 
in  part  lives  on  its  vegetation.  There  are,  for  example,  among  the 
less  accessible  cliffs  various  “bocks,”  baboons,  etc.,  which  range 
through  the  canyon  and  go  to  the  canyon  floor  near  its  mouth  for 
water. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


67 


The  floor  of  the  canyon,  as  well  as  the  sides,  are  well  covered  with 
vegetation,  which,  however,  appears  to  be  more  abundant  on  the 
south  (?)  side  than  on  the  north  side  and  most  abundant  on  the  floor. 

The  number  of  trees  and  shrubs  in  the  canyon  is  apparently  con¬ 
siderable.  By  the  water  is  the  arboreal  species  Rhus  lancea ,  willow¬ 
like  in  appearance,  and  Lycium  sp.,  and  near  the  stream  are  Gym - 
nosporia  buxifolia,  Mesembryanthemum  unidens,  Senecio  longifolius 
(plate  13),  Sutherlandia  frutescens,  and  others. 

On  the  south  (?)  side  of  the  canyon  the  woody  perennials  are  mainly 
about  1  meter  in  height  and  largely,  if  not  exclusively,  sclerophylls. 
There  are  no  trees.  Among  the  species  seen  are  the  following :  Buphane 
distacha,  Cussonia  spicata,  Cadaba  juncea ,  Cotyledon  decussata,  Crassula 
perfossa,  Eriocephalus  sp.,  Euclea  undulata,  Grewia  sp.,  Hermannia 
canescens,  Lobostemon  sp.,  Othonna  pavonia,  Pachypodium  bispinosum , 
Pelargonium  tatragonum,  Pteronia  incana,  Rhus  sp.,  Senecio  cotyledonis , 
S.  longifolius ,  Stachys  sp.,  and  Thesium  sp. 

Of  these  species,  Cussonia  occurs  only  at  the  immediate  base  of  the 
escarpment  and  in  few  numbers,  and  the  balance  are  apparently  less 
restricted,  both  as  to  distribution  and  abundance. 

On  the  opposite  side  of  the  canyon  there  appears  to  be  a  much  larger 
proportion  of  succulents,  especially  of  the  leaf-succulent  type,  and, 
as  before  mentioned,  the  vegetation  does  not  seem  to  be  so  abundant. 
Among  other  species  occurring  on  this  side  are  the  following:  Bulbine 
rostrata,  Cotyledon  orbiculata,  Crassula  perfossa ,  Mesembryanthemum 
angulatum,  M.  haworthii,  M.  nobile,  and  Pachypodium. 

Many  of  the  genera  to  be  found  in  the  vicinity  of  Beaufort  West  also 
occur  in  the  Upper  Karroo,  where  the  Composite  appear  to  be  es¬ 
pecially  well  represented.  On  the  other  hand,  as  will  be  seen  from 
the  short  species  lists  given  above,  there  are  many  succulents  as  well 
as  sclerophylls  in  this  region,  by  the  former  of  which  it  establishes  its 
Karroid  nature.  The  flora  of  the  immediate  region  may  possibly 
be  intermediate,  therefore,  between  that  of  the  Central  and  of  the 
Upper  Karroo. 

Prince  Albert  Road. 

Prince  Albert  Road,  altitude  2,012  feet,  is  74  miles  southwest  of 
Beaufort  West  and  lies  fairly  in  the  central  part  of  the  Central  Karroo. 
The  rainfall,  average  of  25  years,  is  4.57  inches  annually,  with  a  mini¬ 
mum  of  3.34  inches.  It  therefore  is  one  of  the  most  arid  stations  of 
the  Karroo. 

Prince  Albert  Road  is  situated  on  a  plain  which  extends  in  a  south¬ 
erly  direction  25  miles,  more  or  less,  to  the  Groote  Zw^arte  mountains, 
but  which  on  the  north  meets  the  foothills  of  the  Nieuweveld  Mountains 
at  no  great  distance.  Excursions  were  made  to  the  low  hills  about 
2  miles  from  town. 


68 


FEATURES  OF  THE  VEGETATION  OF  THE 


On  the  flats  by  the  village  Hibiscus  mens  occurs.  The  species  has 
the  habit  of  a  cucurbit,  for  which  it  could  be  easily  taken.  Marloth 
makes  the  statement  that  the  leaves  are  thickly  covered  with  tri- 
chomes,  that  the  species  succeeds  in  the  most  arid  situations,  and  comes 
into  flower  in  the  middle  of  summer.1  The  plant  is  remarkable  from 
the  fact,  among  other  features,  that  the  leaves  are  actually  large  and 
the  total  leaf-surface  of  the  plant  extensive,  which  would  hardly  be 
expected  in  so  arid  a  habitat. 

Beyond  the  Hibiscus  habitat  the  plain  insensibly  merges  into  a 
poorly  defined  wash,  beyond  which  are  narrow  and  flat-topped  hills, 
which  in  turn  retreat  to  the  mountainous  background,  leaving  narrow 
and  long  vallej^s  between.  In  and  along  the  wash  are  Acacia  karroo , 
leafless  in  June- August,  Carissa  ferox,  and  Tamarix  sp.  There  also  are 
various  species  of  grass. 

Beyond  the  wash  are  low  and  small  rounded  hillocks  on  which 
are  scattering  cushion-like  and  small  succulents,  Mesembryanthemum 
calamiforme  and  Cotyledon  hemispheerica  (?),  with  bare  ground  between 
(plate  11).  In  a  quadrat  10  by  10  meters,  175  individuals,  mainly 
of  these  two  species,  were  counted. 

The  vegetation  of  the  long  hills  and  valleys  is  varied  and  fairly 
abundant,  and  includes  among  others  the  following  species: 


Aster  sp. 

Berkheya  obovata. 
Cotyledon  decussata. 

C.  hemisphserica. 

C.  wallichii. 

Crassula  lycopodioides. 

C.  perfossa. 

C.  tetragona. 

Dicoma  diacanthoides  (?). 
Galenia  africana. 
Geruleum  bipinnatum. 
Geigeria  passerinoides. 
Hermannia  sp. 


Lycium  sp. 

Mesembryanthemum  ana- 
tomicum. 

M.  brevifolium. 

M.  calamiforme. 

M.  croceum. 

M.  crystallinum. 

M.  floribundum. 

M.  haworthii. 

M.  junceum. 

M.  magnipunctatum. 

M.  spinosum. 

M.  splendens. 


M.  uncinatum. 

M.  unifiorum. 

M.  viride. 

Monechma  sp. 

Pteronia  sp. 

Rhus  sp. 

Royena  pallena. 
Sarcocaulon  sp. 
Sutherlandia  frutescens. 
Tetragonia  sp. 

Tripteris  sinuata. 


There  are  in  addition  numerous  other  species,  including  grasses, 
although  the  latter  are  not  abundant. 

It  will  be  seen  that  succulents  are  prominently  represented  at  Prince 
Albert  Road.  This  is  especially  true  of  the  slopes  and  hills. 


Matjesfontein. 

Metjesfontein,  altitude  2,955  feet,  has  a  rainfall  of  6.88  inches 
annually,  and  although  thus  the  precipitation  is  no  greater  than  at  many 
Karroo  stations  farther  east,  the  vegetation,  immediately  about  the 
town  at  least,  is  probably  not  to  be  considered  typically  karroid.  The 
situation  of  the  station  is  in  the  extreme  westerly  §dge  of  the  Central 
Karroo. 


1  Das  Kapland,  l.  c.,  p.  220. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


69 


The  typography  is  various.  The  Wittebergen  are  to  the  south,  and 
a  western  continuation  of  the  Nieuweveld  Range  is  to  the  north.  The 
former  is  possibly  5  miles  distant  and  the  outliers  of  the  latter  not  over 
2  miles  away.  Between  is  in  effect  a  wide  valley  which  on  the  east 
descends  fairly  rapidly  to  Laingsburg,  2,167  feet  altitude,  18  miles 
distant,  and  on  the  west  ascends  rapidly  to  Pietermeintjes,  distant  10 
miles,  altitude  3,585  feet.  At  Laingsburg  the  annual  rainfall  is  4.62 
inches,  while  at  Pietermeintjes  it  is  13.8  inches.  Although  the  annual 
rainfall  at  Matjesfontein  is  relatively  small,  the  topography  of  the 
vicinity  and  its  relation  to  the  highlands  both  to  the  north  and  to  the 
south,  as  well  as  to  the  higher  valley  west,  make  it  altogether  possible 
that  the  valley  vegetation  at  least  is  largely  determined  by  the  telluric 
waters. 

The  vegetation  at  and  in  the  neighborhood  of  Matjesfontein  is 
exceedingly  varied,  the  species  are  many,  and  the  plant  population  as  a 
rule  large.  Both  sclerophylls  and  succulents  are  well  represented 
and  there  are  numerous  bulbous  species.  Only  a  brief  account  can 
be  given  here  of  the  vegetation,  mainly  for  the  purpose  of  forming  a 
background  for  the  discussion  of  studies  on  the  transpiration  of  a  few 
species  which  were  carried  on  at  Matjesfontein,  Whitehill,  and  Tweed- 
side. 

The  leading  physiographic  formations  in  the  immediate  neighbor¬ 
hood  of  Matjesfontein  appear  to  be  (1)  stream-way,  (2)  plain  or  valley 
floor,  (3)  rocky  outcrops,  (4)  kopjes,  and  (5)  kopje  slopes.  Of  these, 
it  need  only  be  said  that  the  kopjes  visited  were  from  about  100  to 
about  1,000  feet  above  the  surrounding  plain.  By  rocky  outcrops 
is  meant  areas  on  the  plain,  often  of  relatively  small  extent,  on  the 
surface  of  which  are  stones,  sometimes  apparently  in  place,  which  rise 
a  few  feet  only  above  the  plain,  or  such  as  occur  along  the  stream-ways. 
This  formation  is  not  sharply  marked,  however,  although  it  is  probably 
of  considerable  extent  when  taken  altogether. 

The  vegetation  along  the  main  stream  is  characterized  by  Rhus 
viminalis,  Acacia  karroo ,  and  Lycium  sp.,  of  which  the  species  of 
Rhus  is  confined  to  the  banks,  while  the  other  species  may  wander  over 
the  flood-plain  or  bottoms  adjacent. 

Some  quadrats  were  made  on  the  valley  floor  and  on  areas  where 
there  was  outcropping  of  rocks.  These  give  sufficiently  well  for  the 
purpose  at  hand  the  nature  of  the  vegetation  of  such  formations. 

An  area  10  by  10  meters  in  size,  situated  about  0.75  mile  west  of  the 
village  of  Matjesfontein  and  well  in  the  valley,  was  studied.  The  soil 
was  somewhat  sandy  and  there  were  no  stones  on  the  surface.  The 
vegetation  consisted  of  sclerophylls,  with  some  bunch-grass  and  many 
small  flowering  species,  bulbous  and  otherwise.  In  this  quadrat  49 
individuals,  sclerophylls,  were  enumerated.  The  dominant  species 
was  Galenia  africana,  with  a  few  Chrysocoma  tenuifolia ,  Pentzia  virgata, 


70 


FEATURES  OF  THE  VEGETATION  OF  THE 


and  Lycium  sp.  There  were  no  succulents.  The  individuals  were 
large  and  the  ground  was  well  covered  by  them. 

Quadrat  No.  2  was  on  the  lower  slope  of  a  kopje  not  far  above  the 
quadrat  just  characterized.  It  was  well  out  of  the  valley  floor,  there 
were  scattering  stones  about,  and  the  soil  was  fairly  coarse.  In  this 
quadrat  345  individuals  were  counted,  all  of  which  were  small  sclero- 
phylls.  Chrysocoma  tenuifolia  was  the  dominating  species,  although 
there  were  many  of  Pentzia  virgata  as  wrell.  Succulents  were  wanting. 
There  were  few  small  flowering  plants  and  few  grasses.  The  soil 
between  the  shrubs  was  mainly  bare. 

The  third  quadrat  (plate  12a)  was  on  a  low,  rocky  outcrop,  situated 
about  a  mile  east  of  the  village  and  in  the  midst  of  the  valley.  The 
soil  was  fairly  coarse  and  there  were  stones  of  various  sizes,  some  pos¬ 
sibly  in  place,  which  appeared  on  the  surface.  There  were  counted  in 
a  representative  area  10  by  10  meters,  330  individuals,  of  which  160 
were  sclerophylls  and  170  were  succulents.  All  of  these  were  small, 
in  part  probably  from  the  effects  of  grazing.  The  following  species 
were  found:  Aster  filifolius,  Cotyledon  orbiculata,  C.  reticulata,  Crassula 
columnaris,  Eriocephalus  sp.,  Helichrysum  ericifolium,  Pelargonium  sp., 
Pteronia  glomerata ,  Stoebe  sp.,  and  Tetragonia  sp. 

Another  quadrat,  No.  4  (plate  21),  where  there  also  were  rocks 
strewn  on  the  surface,  and  possibly  some  in  place,  was  studied  about 
1.5  miles  to  the  north  of  Matjesfontein.  This  was  a  slope  not  far 
from  a  kopje,  although  it  appeared  to  be  quite  independent  of  the 
latter.  Here  397  individuals  were  counted  in  an  area  of  the  same  size 
as  before,  of  which  213  were  succulents.  Mesembryanthemum  spino- 
sum  dominated,  but  of  the  sclerophylls  Pentzia  virgata  was  most 
numerous.  There  was  hardly  a  sclerophyll  but  what  had  one  or 
more  small  succulents  beneath.  Among  the  succulents  were  two 
species  of  Mesembryanthemum  with  cushion-like  habit,  Crassula 
columnaris,  and  others.  Asparagus  capensis  was  often  associated 
with  Pentzia  virgata. 

In  quadrat  No.  5,  which  was  on  the  plain,  there  were  large  sclero¬ 
phylls.  Here  were  Chrysocoma  tenuifolia,  Elytropappus  rhino - 
cerotis,  Galenia  africana,  and  Mesembryanthemum  spinosum.  Mesem¬ 
bryanthemum  dominated. 

Different  species  dominate  in  different  portions  of  the  veld  by  Mat¬ 
jesfontein,  as  has  already  been  mentioned.  Although  this  may  be 
seen  in  several  places,  it  was  especially  noted  in  an  area  to  the  south 
of  but  not  distant  from  the  village;  the  water  conditions  were  appar¬ 
ently  especially  favorable,  as  the  plain  was  a  bajada  coming  from 
the  hills  to  the  south.  At  the  place  referred  to  Elytropappus  rhino - 
cerotis  either  dominates  or  in  places  constitutes  the  only  species. 
The  plants  are  often  1.5  meters  high.  A  feature  of  the  Elytropappus 
habitat  is  the  lack  of  smaller  plants  clustered  about  the  base  of  the 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


71 


larger  and  dominant  species,  growing  under  their  11  protection/’  as 
is  so  commonly  the  case  in  the  Karroos. 

The  flora  of  the  kopjes  is  especially  rich  both  as  to  numbers  and 
species.  Here,  among  other  species,  occur  Aloe  striata  (7),  Cotyledon 
coruscans,  C.  orbiculata,  C.  paniculata,  Crassula  per} ossa  (plate  16), 
Euclea  undulata,  Euphorbia  mauritanica  (plate  17),  Lebeckia  psiloloba 
(plate  20),  Mesembryanthemum  junceum  (plate  15),  Mesembryanthe- 
mum  sp.,  and  Rhus  sp. 

In  limited  areas  in  the  immediate  neighborhood  of  Matjesfontein, 
but  over  large  areas  in  the  valley  about  8  miles  north,  there  occur 
Mesembryanthemum  sp.  in  cushion-like  habit,  almost  to  the  exclusion 
of  other  species.  At  Majesfontein  there  are  rarely,  if  at  all,  mono- 
specific  communities,  although  in  certain  instances,  as  that  just  given, 
certain  growth-forms  may  be  the  rule.  Over  limited  areas  also  a  single 
species  may  constitute  80  per  cent  of  the  entire  population,  as,  for 
example,  about  1.5  miles  north  of  town,  where  on  a  gentle  slope 
Mesembryanthemum  spinosum  dominates.  The  shoot  of  this  species 
is  much  branched,  and  the  small,  fleshy  leaves  are  numerous,  so  that 
the  ground  beneath  is  well  shaded.  Possibly  for  this  reason,  although 
the  shallow  placing  of  the  roots  may  be  contributory,  the  species 
usually  occurs  singly.  When  Galenia  africana  is  associated  with  it, 
the  latter  is  often  small,  but  when  it  is  with  Elytropappus  rhino - 
cerotis  both  appear  equally  vigorous.  In  the  case  of  Galenia ,  as  will 
be  mentioned  elsewhere,  it  is  possible  that  the  transpiring  power  is 
especially  large  and  that  an  abundant  water-supply  is  essential,  which 
might  not  be  possible  to  obtain  were  the  species  closely  associated 
wdth  M.  spinosum. 

There  are  many  species  at  and  in  the  vicinity  of  Matjesfontein, 
some  of  which  are  scattered  here  and  there  about  the  valley,  while 
others  are  of  even  more  restricted  distribution.  Of  such  species  the 
following,  some  of  which  are  illustrated  in  plates  13  to  31,  may  be 
mentioned : 


Aloe  variegata. 
Anacampseros  papyracea. 
Buphane  disticha. 
Cotyledon  mamillaris. 

C.  orbiculata. 

C.  paniculata. 

C.  reticulata. 

C.  wallichii. 

Crassula  columnaris. 

C.  lycopodioides. 

C.  perfossa. 

C.  portulacea. 

C.  tetragona. 

Euclea  undulata. 
Euphorbia  hystrix  (?). 


E.  multiceps. 

E.  stolonifera. 

Euryops  lateriflorus. 

E.  tenuissimus. 

Haworthia  margaritifera  (?). 
Hyobanche  glabrata. 
Lebeckia  psiloloba. 
Loranthus  glaucus. 
Mesembrjranthemum  crys- 
tallinum. 

M.  junceum. 

M.  pygmaeum. 

M.  spinosum. 

M.  quadrifidum. 
Pelargonium  alternans. 


P.  crithmifolium. 
Pentzia  virgata. 

Protea  neriifolia. 
Pteronia  flexicaulis. 

P.  glomerata. 

P.  incana. 

P.  pallens. 

Relhania  squarrosa. 
Rhus  viminalis. 

Stapelia  pillansii. 

Sutera  ccerulea. 
Sutherlandia  frutescens. 
Thesium  horridum. 

T.  spinosum. 


72 


FEATURES  OF  THE  VEGETATION  OF  THE 


Notes  on  Root  Habits. 

Some  notes  were  made  on  the  root  habits  of  several  perennials 
which  occur  naturally  at  and  in  the  vicinity  of  Matjesfontein.  The 
species  studied  were  both  sclerophylls  and  succulents.  The  habitats 
were  mainly  the  plain  and  along  the  stream- way.  The  following  is  a 
brief  statement  of  the  leading  results. 

Cotyledon  coruscans  (plate  24b,  24c)  was  observed  on  the  veld,  plain, 
near  the  village.  The  root  system  is  poorly  developed  and  super¬ 
ficial.  In  the  specimen  studied  the  main  root  forked  near  the  crown, 
but  there  were  relatively  few  rootlets. 

The  specimens  of  Cotyledon  reticulata  examined  were  growing  near 
C.  coruscans.  This  succulent  is  about  15  cm.  high  and  has  branches 
5  cm.,  more  or  less,  in  diameter.  There  is  apparently  no  tap-root, 
but  several  roots  of  equal  rank  arise  from  the  base  of  the  stem.  They 
take  a  horizontal  course,  keeping  always  near  the  surface  of  the  ground. 
The  roots  branch  frequently  and  there  are  numerous  filamentous 
rootlets. 

The  specimen  of  Cotyledon  wallichii  (plates  16  and  18)  examined 
occurred  on  the  veld  where  stones  were  scattered  on  the  surface.  The 
soil  was  fairly  deep,  however,  and  much  exceeded  the  depth  to  which 
the  roots  penetrated.  The  specimen  was  mature,  the  shoot  being 
about  35  cm.  high.  There  were  two  kinds  of  roots,  of  which  one  sort 
reached  as  far  as  82  cm.  from  the  base  of  the  stem.  This  was  com¬ 
posed  of  five  roots,  each  of  which  was  about  1.8  cm.  in  diameter  at 
the  base.  The  extreme  depth  attained  by  them  was  7.5  cm.  The 
second  type  of  roots  were  short  and  numerous,  and  arose  from  the  root 
crown.  They  bore  groups  of  filamentous  roots  which  had  the  appear¬ 
ance  of  being  short-lived. 

Crassula  lycopodioides  (plate  30b)  has  a  meager  root-system.  The 
roots  are  few  and  all  superficial.  None  were  found  over  10  cm.  deep, 
and  for  the  most  part  they  were  within  5  cm.  of  the  surface. 

Elytropappus  rhinocerotis  is  fairly  abundant  on  the  plain  at  Mat¬ 
jesfontein  and  becomes  increasingly  so  as  one  goes  west,  where  the 
rainfall  is  greater.  The  root-system  of  the  species  was  not  especially 
studied,  although  observations  were  made  in  certain  instances.  There 
is  a  well-marked  development  of  superficial  roots  (plate  27) ;  some  of 
these  are  large  and  give  rise  to  shoots,  as  in  the  case  of  Lycium  sp. 

Euphorbia  multiceps  (plate  25a,  25b)  occurs  at  Matjesfontein  on 
the  valley  floor  and  the  specimens  examined  were  situated  where  the 
soil  was  relatively  deep.  The  main  root  is  especially  well  developed 
and  in  the  specimens  examined  appeared  to  strike  deeply  and  did 
not  send  out  superficial  horizontal  roots.  Where  the  main  root  was 
forked  the  two  branches  continued  to  go  downward.  This  an  unusual 
type  of  root-system  for  a  succulent. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


73 


An  undetermined  Euphorbia  with  growth  habit  much  like  E. 
mauritanica  and  which  was  growing  in  a  rocky  place  on  the  veld  had  a 
short  and  succulent  or  at  least  stocky  tap-root  which  penetrated  the 
soil  10  cm.  and  then  branched,  and  the  latter  continued  downward. 
There  appeared  to  be  no  superficially  placed  horizontal  roots. 

Euryops  lateriflorus  occurs  at  Matjesfontein  in  a  rocky  outcrop 
by  the  stream  near  the  village,  where  the  transpiration  studies  on 
the  species  were  carried  out,  but  the  roots  were  examined  at  a  place 
7  miles  west,  where  the  soil  is  deeper  and  the  rainfall  greater  (plate  23) . 
In  the  immediate  vicinity  of  the  plants  examined  were  found  Ely- 
tropappus  rhinocerotis  and  other  sclerophyllous  shrubs,  in  addition  to 
Euryops. 

The  specimens  were  growing  on  the  edge  of  a  shallow  “box”  canyon 
and  the  roots  had  been  partly  exposed  by  erosion.  The  root  system 
is  characterized  by  a  pronounced  tap-root  and  quite  as  pronounced 
laterals  which  are  horizontally  placed.  The  tap-root  was  over  40 
cm.  long.  The  laterals  arose  about  5  cm.  beneath  the  surface  of  the 
ground  and  extended  for  a  distance  of  32  cm.,  more  or  less,  maintaining 
about  the  same  depth  throughout  their  course.  Another  specimen 
had  the  same  character  of  roots,  but  the  main  root  was  found  to  pene¬ 
trate  somewhat  more  deeply. 

Galenia  africana  is  very  abundant  on  the  flats  by  the  stream  at 
Matjesfontein.  Where  the  roots  were  examined  the  banks  were  from 
2  to  4  meters  above  the  water  itself  and  the  soil  had  been  eroded, 
leaving  perpendicular  walls.  Plate  22  illustrates  the  character  of  the 
roots  of  the  species.  There  is  a  well-defined  main  root  which  may 
attain  to  a  depth  of  3  meters  or  more.  The  secondary  and  tertiary 
roots  are  numerous.  Important  branches  go  fairly  directly  downward. 
Many  small  roots  arise  at  the  crown  of  the  main  root  and  extend 
outward  horizontally  for  1  meter,  more  or  less.  They  are  essentially 
superficial.  At  a  depth  of  about  50  cm.  other  laterals  arise  and  take 
a  fairly  horizontal  direction  in  growth.  Deeper  than  this  the  laterals 
are  not  so  numerous,  and  they  appear  to  assume  a  more  nearly  vertical 
position  than  those  more  superficially  placed.  The  root-system  of  the 
species,  therefore,  is  especially  well  developed.  It  penetrates  deeply 
and  reaches  out  widely. 

In  Galenia  each  plant  appears  to  arise  de  novo  and  not  from  under¬ 
ground  organs  of  any  kind. 

The  specimens  of  Lycium  sp.  (plate  27)  whose  root-systems  were 
examined  were  growing  in  the  vicinity  of  Galenia  africana  last  described 
and  under  the  same  soil  conditions.  The  species  has  a  pronounced 
tap-root  which  penetrates  the  ground  2  meters  or  more.  The  main 
root  does  not  branch  freely.  At  a  depth  of  about  50  cm.  several  large 
laterals  arise  and  go  nearly  directly  downward.  There  are  few  rela¬ 
tively  small  laterals  near  the  crown  of  the  main  root,  and  one  or  more 


74 


FEATURES  OF  THE  VEGETATION  OF  THE 


prominent  laterals  30  cm.,  more  or  less,  beneath  the  surface.  Im¬ 
mediately  below  the  surface  of  the  ground  were  one  or  more  large 
superficial  roots  which  gave  rise  on  the  one  hand  to  numerous  roots 
which  went  directly  downward,  and  on  the  other  hand  to  branches. 
Vegetative  reproduction  appears  to  be  a  well-established  method  of 
propagation  in  this  species. 

Mesembryanthemum  junceum  (plate  26)  is  a  much-branched  shrub 
bearing  small  fleshy  leaves  and  occurs  on  the  plain  in  the  vicinity  of 
Matjesfontein,  often  in  association  with  Galenia  africana.  The 
roots  appear  to  be  mainly  superficial.  In  one  specimen  which  was 
examined  13  laterals  arose  about  6  cm.  beneath  the  surface  and  ex¬ 
tended  outward,  maintaining  fairly  closely  this  depth,  and  another  but 
somewhat  smaller  plant  of  the  same  species,  having  a  shoot  15  cm. 
high,  had  a  well-marked  main  root  which,  tapering  rapidly,  forked  at 
a  depth  of  12  cm.  Several  laterals  arose  near  the  surface  of  the  soil 
and  extended  in  a  horizontal  direction  for  a  distance  of  30  cm.  or  more. 

Mesembryanthemum  spinosum  (plate  26)  also  occurs  on  the  plain  and 
appears  to  have  a  root-system  resembling  that  of  the  species  last 
described,  in  that  it  is  mainly  superficial.  There  is  a  main  root  which 
does  not  appear  to  penetrate  deeply.  Numerous  laterals  are  given 
off  near  the  surface,  which  in  one  instance  were  found  to  extend  to  the 
base  of  the  neighboring  plants,  Cotyledon  wallichii  and  Galenia  africanay 
about  1.25  meters  distant.  As  regards  both  species,  it  is  possible 
that  under  appropriate  conditions  of  soil  and  of  soil  moisture  deep  root 
penetration  might  take  place. 

In  Pelargonium  crithmifolium  (plate  31b)  which  occurs  on  low 
and  somewhat  stony  hills  near  the  village,  there  is  a  stout  and  short 
stem  and  a  much-enlarged  root  crown  with  well-defined  main  root. 
There  do  not  appear  to  be  many  laterals,  and  none  which  lie  close  to 
the  surface  of  the  soil.  The  root-system  can  be  characterized  as 
being  poorly  developed  and  meager. 

Other  species  of  Pelargonium  of  similar  growth  habit  in  the  same 
vicinity  had  the  character  of  root  as  just  briefly  described.  In  one 
specimen  where  the  root  crown  measured  2.5  by  5  cm.  in  cross-section, 
the  main  root  was  traced  into  the  stony  soil  to  a  depth  exceeding 
18  cm.,  during  which  distance  no  laterals  were  given  off.  The  depth 
attained  by  the  root  was  not  learned.  However,  it  appears  probable 
that  in  the  vicinity  of  Matjesfontein  the  roots  of  species  of  Pelargo¬ 
nium  of  this  type  do  not  have  superficial  roots,  but  that  on  the  con¬ 
trary  they  are  fairly  deeply  placed.  A  similar  condition  was  noted 
also  in  Euphorbia  multiceps.  Both  of  these  species  with  water-storage’ 
capacity  thus  constitute  an  apparent  exception  to  the  frequently 
observed  root  habit  of  succulents. 

Salsola  aphylla  was  observed  on  the  flats  back  of  the  Karroo  Botan¬ 
ical  Gardens,  Whitehill,  where  the  roots  had  been  exposed  along  a 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA.  75 

narrow  wash  by  erosion.  The  root  system  was  characterized  by  a  well- 
developed  tap-root  whose  depth  exceeded  45  cm.  The  few  laterals 
were,  for  the  most  part,  about  5  cm.  beneath  the  surface  of  the  ground. 
They  extended  26  cm.  or  more  away  from  the  main  root.  In  one 
instance  such  a  lateral  was  seen  to  turn  and  run  directly  downward 
45  cm. 

Stapelia  pillansii  (plate  28d),  which  occurs  on  a  small  rocky  out¬ 
crop  about  0.5  mile  east  of  Matjesfontein,  has  short  and  much- 
branched  roots,  all  of  which  appear  to  be  shallowly  placed.  They 
extend  from  the  plant  cluster  only  a  few  centimeters.  The  root- 
system,  therefore,  is  poorly  developed. 

Rhus  viminalis  (plate  20)  is  confined  to  the  immediate  vicinity  of 
the  stream  at  Matjesfontein.  It  therefore  occasionally  happens 
that  the  roots  are  exposed  through  the  washing  away  of  the  banks. 
About  a  mile  east  of  Matjesfontein  there  has  been  marked  and  recent 
erosion  along  the  stream-way.  In  one  instance  several  roots  of  Rhus 
were  laid  bare.  In  this  case  there  appears  to  have  been  a  marked 
root  development  at,  or  not  far  above,  the  water-level.  For  example, 
one  root  was  traced  more  than  25  meters,  and  for  most  of  the  way  it  lay 
about  1.5  to  2  meters  beneath  the  surface  of  the  stream-bank.  There 
was  a  well-developed  main  root,  the  depth  of  the  penetration  of  which, 
however,  was  not  learned. 

From  the  failure  to  find  species  with  succulent  roots,  other  than 
certain  bulbous  forms,  it  is  not  to  be  concluded,  however,  that  such 
are  wanting  in  the  vicinity  of  Matjesfontein  or  of  Beaufort  West, 
where  most  of  the  studies  on  the  plants  in  the  field  were  carried  out. 
Thus  Pachypodium  bispinosum,  which  was  found  growing  on  the  side 
of  a  gorge  6  miles  east  of  the  latter  place,  but  which  was  not  especially 
examined,  is  known  to  have  tuberous  roots.  Marloth1  figures  the 
enlarged  roots  of  this  species  as  well  as  those  of  other  species.  An 
apparent  exception  to  the  first  statement  is  to  be  found  in  Haworthia 
sp.  (plate  29a),  in  which,  as  will  be  seen,  the  shallowly  placed  and 
radiating  roots  are  somewhat  tuberous.  In  this  species  the  shoot  is 
not  succulent,  so  that  a  succulent  condition  of  the  roots  may  be  a 
specific  quality.  Succulency,  both  of  root  and  of  shoot  in  the  same 
species,  does  not  seem  to  occur.  It  can  be  concluded,  therefore, 
that  root  succulency  is  not  common  at  Matjesfontein  and  probably 
not  at  Beaufort  West. 

Summary. 

It  will  be  seen,  therefore,  that  the  roots  of  the  plants  examined  can 
be  conveniently  grouped  into  those  with  a  well-developed  main  root, 
as  in  Euphorbia  multiceps  and  Pelargonium  crithmifolium;  those  with 
both  main  root  and  laterals  both  well  developed,  as  in  Elytropappus, 


1  Das  Kapland,  l .  c.,  p.  316. 


76 


FEATURES  OF  THE  VEGETATION  OF  THE 


Euryops,  Galenia,  Lycium,  Salsola ,  and  Rhus  viminalis ;  and  those 
with  superficial  roots,  as  in  Cotyledon,  Crassula,  certain  species  of 
Mesembryanthemum,  and  Stapelia. 

Euphorbia  multiceps  and  Pelargonium  crithmifolium  are  succulents 
and  as  such  would  be  expected  to  have  poorly  defined  main  root,  with 
laterals  originating  near  the  surface  of  the  ground,  or  with  roots  of 
equal  rank,  but  in  any  event  with  roots  which  do  not  penetrate  deeply. 
In  such  position  advantage  is  taken  of  very  light  rains.  In  the  case 
of  these  species,  however,  so  far  as  the  observations  extended,  it 
appeared  that  even  when  the  downward  progress  of  the  main  root  is 
turned,  or  stopped  by  rocks,  the  type  of  the  root  nevertheless  does  not 
become  changed.  The  prominent  development  of  the  main  root  in 
the  two  species  appears,  therefore,  to  be  obligate. 

The  balance  of  species  with  succulent  habit,  either  of  stem,  leaf, 
or  root,  have  as  a  rule  a  meager  root-system.  In  some  species  the  roots 
reach  out  but  a  few  centimeters  from  the  base  of  the  plant,  but  in  others 
they  extend  so  far  as  82  or  125  cm.  In  any  event,  they  lie  within  a 
few  centimeters  of  the  surface. 

The  sclerophyllous  species  all  appear  to  have  roots  which  vary  to 
a  certain  and  possibly  usually  considerable  degree  as  regards  the  depth 
and  lateral  extent  of  development,  but  they  all  agree  on  having  rela¬ 
tively  large  development  of  the  root-system.  In  Elytropappus  rhino - 
cerotis  the  superficial  roots  are  a  prominent  feature,  and  in  Galenia 
africana  there  are  many  small  roots  about  the  base  of  the  stem,  but  in 
both  species  there  are  also  deeply  penetrating  roots.  Observations 
indicate  that  Elytropappus  may  occur  to  the  exclusion  of  other  species, 
especially  small  succulents  which  often  are  found  gathered  about  the 
base  of  sclerophylls.  The  presence  of  numerous  superficial  roots  in  the 
species  may  be  a  contributing  factor  to  this  condition.  That  there 
may  in  fact  be  some  sort  of  distributional  adjustment  between  species 
on  the  basis  of  difference  in  root  types,  or  possibilities  in  the  direction 
of  development,  seems  not  unlikely.  Thus  there  is  often  an  associa¬ 
tion  of  shallowly  rooted  and  of  deeply  rooted  species,  as  mentioned 
above.  Where  species  requiring  a  good  supply  of  water,  such  as  the 
sclerophylls  in  question,  are  closely  associated,  the  effects  of  the  sub¬ 
terranean  competition  may  be  evident  from  the  poor  shoot-growth 
on  the  part  of  either,  or  both.  Thus  it  was  noted  that  when  Galenia 
africana  was  associated  with  Mesembryanthemum  spinosum,  the  shoot 
of  the  former  is  often  small,  but  when  Elytropappus  occurs  in  associa¬ 
tion  with  the  Mesembryanthemum  the  shoot  development  of  the  species 
(Elytropappus)  is  apparently  quite  normal.  As  all  of  these  species 
have  different  types  of  generalized  roots,  there  is  possibly  some  sort  of 
mutual  adjustment  and  accommodation  in  the  last-named  instance, 
which  is  lacking  in  the  one  first  referred  to,  and  this  may  well  be  in 
connection  with  the  roots. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


77 


In  Lycium  sp.  and  Elytropappus  rhinocerotis  shoots  arise  from  stolon¬ 
like  roots,  which  is  probably  to  be  considered  an  important  way  of 
winning  territory  and  of  holding  that  already  occupied,  as  well  as  of 
repelling  invading  species. 

SOME  FEATURES  OF  FOLIAR  STRUCTURE. 

The  results  of  studies  on  the  inner  morphology  of  the  perennials 
of  foreign  regions,  particularly  of  central  Australia,  led  the  writer  to 
examine  that  of  some  of  the  plants  observed  by  him  in  southern 
Africa.  Not  all  of  the  plants  studied  in  the  field,  however,  were  used, 
for  the  following  reasons:  In  case  of  material  which  was  collected  and 
immediately  placed  in  suitable  reagents,  or  of  living  material,  and 
there  was  a  little  of  each,  there  was  no  difficulty,  but  where  the  material 
available  was  dried,  as  for  the  most  part  was  the  case,  the  matter  was 
quite  different.  In  the  last  event  only  species  with  a  large  amount  of 
supporting  tissue  could  be  utilized,  so  that  much  of  it — all  in  fact  that 
was  not  available  in  the  conditions  first  referred  to — had  to  be  discarded. 
These  facts  are  of  interest  for  the  reason,  as  will  be  referred  to  later, 
that  under  arid  conditions  the  relative  amount  of  cell-wall  stuff  organ¬ 
ized  by  the  plant  may  be  very  large,  but  under  less  arid  conditions  the 
type  of  metabolism  is  quite  different,  possibly  leading  to  the  ultimate 
formation  of  mucilages.  Also,  leaves  of  essentially  mesophytic  struc¬ 
ture  are  not  well  suited  in  the  dry  form  for  such  studies.  These  facts 
have  worked  to  materially  limit  the  scope  of  the  structural  studies. 
The  studies  are  further  limited  by  the  fact  that  only  leaves  of  winter 
were  used,  so  that  such  forms  as  were  in  leaf  in  summer,  as,  for  example, 
the  acacias,  could  not  be  utilized. 

The  limitations  in  material  have  made  it  advisable  to  confine  the 
treatment  of  structure  to  leaves,  or  at  least  to  chlorophyll-bearing 
organs.  Begun  in  the  first  place  on  species  which  were  used  in  the 
studies  on  foliar  transpiring  power,  the  work  was  extended  so  that  it 
finally  included  all  material  suitable  for  examination.  This  material, 
it  should  be  remarked  in  passing,  had  been  studied  in  various  ways, 
other  than  that  mentioned  above,  in  the  field. 

Leaves  of  perennials  of  an  arid  region  usually  exhibit  features  which 
may  not  only  be  characteristic  of  the  species,  but  of  xerophytes  in 
general.  In  spite  of  this,  however,  it  is  doubtful  if  any  foliar  structures 
of  species  of  an  arid  region  are  really  “new,”  that  is,  quite  character¬ 
istic  of  them  and  of  no  others.  The  differences  are,  perhaps,  rather 
of  quantity  than  of  quality,  although  the  quantitative  differences  may 
be  great  indeed.  There  need  only  be  called  to  mind  the  formation 
of  a  heavy  outer  epidermal  wall,  or  of  much  supporting  tissue,  or  the 
frequent  deep  placing  of  stomata,  or  the  formation  of  palisades,  or  the 
organization  of  mucilages,  not  to  extend  the  list.  But  there  is  the 
background  of  heredity  on  which  present-day  forces  play  and  which 


78 


FEATURES  OF  THE  VEGETATION  OF  THE 


makes  the  development  in  any  given  direction  possible.  This  will  be 
mentioned  again  later.  But  it  is  doubtless  on  account  of  such  a  con¬ 
servative  factor  that  there  is  the  apparent  limitation  as  to  structure 
above  referred  to;  and,  finally,  regarding  organs  which,  like  leaves,  are 
directly  affected  by  the  environment,  it  is  rather  a  matter  of  surprise 
that  so  much  and  not  so  little  of  possible  ancestral  qualities  are  to  be 
found  in  them  to-day. 

An  examination  was  made  of  the  leaf  structure  of  the  following 
species : 


Aloe  variegata. 
Antizoma  capensis. 
Asclepias  filiformis  (?). 
Asparagus  striatus. 
Bauhinia  marlothii. 
Cadaba  juncea. 

Carissa  ferox. 
Cotyledon  paniculata. 
Cussonia  spicata. 


Eriocephalus  sp. 

Euclea  undulata. 
Euryops  lateriflorus. 
Galenia  africana. 
Grewia  cana. 
Gymnosporia  buxifolia. 
Pentzia  virgata. 

Protea  neriifolia. 
Pteronia  flexicaulis. 


Pteronia  incana. 
Relhania  squarrosa. 
Royena  pallens. 

Rhus  viminalis. 

Rhus  sp. 

Stachys  sp. 

Stcebe  sp. 

Sutherlandia  frutescens. 


NOTES  ON  LEAF  STRUCTURE. 

Aloe  variegata. 

The  material  of  Aloe  variegata  which  was  examined  was  living  and 
had  been  collected  in  the  vicinity  of  Matjesfontein. 

Two  leaves  of  the  same  plant  were  studied,  of  which  one  was  older 
and  smaller  and  was  placed  near  the  base  of  the  rosette,  and  the  other 


Fig.  8. — a.  Cross-section  of  leaf  of  Aloe  variegata  to  show  the  heavy  outer  epidermal  wall,  with 
the  cuticularized  portion  indicated  by  dotted  lines,  and  the  deeply  placed  stomata. 
X300. 

b,  Asparagus  striata ,  cro33-section  of  leaf,  showing  the  heavy  epidermis  with  thickened 

outer  wall,  the  limited  development  of  palisades,  and  a  portion  of  a  centrally 
situated  mass  of  sclerenchymatous  tissue.  X300. 

c,  Protea  nerriifolia,  section  of  an  old  leaf,  in  which  is  indicated  the  heavy  epidermis 

with  much  thickened  outer  wall  and  deeply  placed  stomata.  The  pronounced 
palisade  formation  is  indicated.  X300. 

d,  Section  of  leaf  of  Antizoma  capensis  showing  the  superficially  placed  stomata  and 

palisade  chlorenchyma.  X300. 

e,  Galenia  africana,  to  show  the  vesicular  epidermal  cells,  superficial  placing  of  the 

stomata,  and  character  of  the  outer  chlorenchyma.  X300. 

/,  Cross-section  of  branch  of  Cadaba  juncea  showing  the  deeply  placed  stomata  with 
marked  development  of  outer  vestibule.  The  heavy  character  of  the  epidermis  is 
indicated.  X300. 

g,  Cadaba  juncea,  shoot,  fragment  of  section  taken  immediately  within  the  chlorenchyma 

to  show  the  tracheids.  X300. 

h,  Cross-section  of  leaf  of  Grewia  cana  showing  the  dorsi-ventral  symmetry  of  structure. 

The  heavy  epidermis  of  the  dorsal  side,  without  cover  trichomes,  and  the  light 
epidermis  of  the  ventral  side  with  trichomes  are  shown. 

i,  Grewia  cana,  fragment  of  cross-section  of  ventral  portion  of  leaf  showing  the  super¬ 

ficially  placed  stomata.  X325. 

j,  Fragment  of  cross-section  of  leaf  of  Rhus  viminalis,  to  show  excessive  development 

of  palisades.  The  thickness  of  the  leaf  is  indicated.  X 150. 

k,  Detail  of  epidermis  from  ventral  side  of  leaf  of  Rhus  viminalis  showing  character 

of  stomata,  and  suggestion  that  of  the  spongy  chlorenchyma  below  it.  X325. 

l,  Rhus  sp.  Whitehill.  Fragment  of  epidermis  from  dorsal  surface  of  leaf,  to  show  the 

heavy  covering  of  resin  and  its  relation  to  the  base  of  trichome.  X300. 

m,  Gymnosporia  buxifolia,  detail  of  epidermis  of  leaf,  longitudinal  section,  showing 

superficial  placing  of  stomata  and  the  fairly  heavy  outer  walls.  Cuboid  sub- 
epidermal  cells,  in  effect  a  hypoderm,  separate  the  palisades  from  the  epidermis. 
X325. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


79 


Fig.  8. 


80 


FEATURES  OF  THE  VEGETATION  OF  THE 


and  larger  leaf,  about  11  cm.  in  length,  was  situated  about  midway 
between  the  youngest  and  the  oldest  leaves.  The  older  leaf  had  a 
somewhat  upwardly  inclined  position,  while  that  of  the  younger  leaf 
was  nearly  vertical.  In  the  former  case  the  dorsal  and  concave  sur¬ 
face  was  fully  exposed,  while  the  ventral  and  convex  surface  was 
protected  against  the  direct  rays  of  the  sun,  but  in  the  case  of  the 
younger  and  larger  leaf  the  ventral  surface  was  exposed  and  the  dorsal 
surface  was  fairly  well  protected  by  the  younger  and  inner  leaves, 
as  well  as  by  the  position  itself. 

A  cross-section  of  the  leaf  shows  a  simple  structure.  For  the 
most  part,  the  tissue  consists  of  large,  cuboid  cells,  with  prominent 
intercellular  spaces,  and  without  chlorophyll.  A  relatively  narrow 
zone  of  chlorenchyma  bounds  the  latter  tissue  on  the  outside.  The 
innermost  cells  of  the  chlorenchyma  are  similar  in  form  and  size  to 
the  colorless  mesophyll  cells,  but  those  nearer  the  epidermis  are 
elongated  somewhat  in  a  direction  at  right  angles  to  the  surface  of  the 
leaf.  They,  however,  are,  properly  speaking,  not  palisades. 

The  cell  contents  of  both  chlorenchyma  and  mesophyll  stain  with 
an  alcoholic  solution  of  safranin  and  probably  contain  mucilage  in 
some  form.  Especial  mucilage-bearing  cells,  however,  were  not 
observed. 

In  the  older  leaf  the  outer  chlorenchyma  was  in  large  part  crushed 
against  the  epidermis,  giving  much  the  effect  of  lifeless  cork  cells. 
Instances  were  observed,  in  this  case,  where  the  inner  and  thick 
(cellulose)  layer  of  the  outer  epidermal  wall  was  apparently  wanting. 

The  epidermis  of  both  the  dorsal  and  the  ventral  surfaces  is  heavy, 
with  an  outer  wall  frequently  more  than  one-half  the  entire  diameter 
of  the  epidermal  cell.  The  cuticle,  and  especially  the  portion  of  the 
outer  wall  which  is  cuticularized,  is  relatively  thick  (fig.  8a).  The 
inner  portion,  which  does  not  stain  with  safranin,  constitutes  most  of 
outer  wall.  * 

Stomata  occur  on  both  leaf-surfaces  and  are  deeply  placed,  owing 
to  the  great  thickness  of  the  outer  epidermal  wall.  A  canal  leads 
through  the  outer  epidermal  wall  to  the  guard-cells.  This  is  some¬ 
what  restricted  at  the  opening,  which  has  a  diameter  of  about 
0.0076  mm.  It  was  found  that  in  many  cases  the  canal  was  filled 
with  a  dark-colored  substance,  not  soluble  in  cold  alcohol. 

Asparagus  striatus. 

The  leaves  of  Asparagus  striatus  were  studied  from  material  col¬ 
lected  on  the  lower  northeast  slope  of  a  kopje  near  Beaufort  West. 
The  leaves  are  short  and  rigid,  having  the  appearance  of  dwarf 
branches,  and  the  dried  plants,  when  properly  treated  for  studying, 
present  the  essential  structural  points  of  the  organ.  Only  dried  plants 
were  used. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


81 


The  most  striking  anatomical  features  of  the  leaves  are  as  follows: 
In  cross-section  the  leaf  is  oval  and  the  following  are  the  most  im¬ 
portant  tissues,  concentrically  arranged.  The  innermost  mesophyll 
is  composed  for  the  most  part  of  fairly  thin-walled  cells,  cuboid  in 
form.  Outside  this  is  a  zone  of  sclerenchyma  almost  devoid  of 
lumen,  about  equaling  the  chlorenchyma  in  thickness.  There  is  then 
a  zone  of  large  cuboid  cells,  two  or  three  cells  in  diameter,  and  either 
within  or  without  this  zone,  somewhat  over  half  the  distance  from  the 
epidermis  to  the  center  of  the  leaf,  occurs  the  region  with  conductive 
tissue.  There  also  are  thin-walled,  parenchymatous  cells,  with 
cellulose  walls,  in  the  zone  last  referred  to.  With  this  exception, 
and  as  shown  by  reaction  with  chloroiodide  of  zinc,  the  walls  of  the 
tissues  mentioned  are  lignified.  Peripheral  to  these  tissues  lies  the 
chlorenchyma,  which  consists  of  an  outer  layer,  one  cell  in  thickness, 
which  is  short-palisade  in  character,  and  an  inner  portion  of  cuboid 
cells.  The  chlorenchyma  is  in  thickness  about  one-eighth  the  diameter 
of  the  leaf.  On  the  ventral  side  of  the  leaf,  however,  palisade  cells 
are  quite  wanting  and  the  subepidermal  layer  of  chlorenchyma  is 
wholly  cuboid. 

Crystal  aggregates,  calcium  oxalate,  occur  in  the  cells  between  the 
chlorenchyma  and  the  supporting  tissue  within,  and  in  the  tissue 
with  lignified  walls  and  relatively  large  lumen  contiguous  to  the  latter. 

Except  in  the  palisades,  intercellular  spaces  are  wanting,  or  rarely 
present. 

The  epidermis  is  fairly  heavy  and  the  outer  wall  is  heavy  and  heavily 
cuticularized.  The  deposition  of  cutin  extends  to  the  lateral  walls, 
the  outer  portions,  at  least,  of  which  give  the  reaction  of  this  substance. 

The  stomata  are  somewhat  deeply  placed  (fig.  8b)  and  the  outer 
wall  of  the  subsidiary  cells  is  projected  slightly,  thus  making  at  the 
same  time  the  canal  leading  to  the  guard-cells  somewhat  longer, 
and  the  latter  of  a  consequence  somewhat  deeper  than  if  the  outer  wall 
of  these  cells  were  level  with  the  general  leaf-surface.  The  thin  outer 
coating  of  the  epidermal  outer  wall,  the  inner  face  of  the  stomatal 
canal,  and  the  outer  guard-cell  ridges,  which  delimit  the  vestibule, 
and  not  shown  in  the  sketch,  color  deep  brick-red  with  chloroiodide 
of  zinc,  indicating  that  they  are  cuticularized. 

It  may  be  emphasized  that  the  leading  structural  characteristics 
of  the  leaf  are  the  large  proportion  of  tissues  of  which  the  walls  are 
lignified,  the  almost  entire  lack  of  intercellular  spaces,  and  the  large 
development  of  supporting  tissue.  These  features  make  for  rigidity 
of  the  leaf. 

Protea  neriifolia. 

The  material  of  Protea  neriifolia  which  was  examined  was  collected 
at  Tweedside,  about  13  miles  west  of  Matjesfontein,  and  at  a  somewhat 
higher  altitude.  Owing  to  the  presence  of  a  relatively  large  amount 


82 


FEATURES  OF  THE  VEGETATION  OF  THE 


of  mechanical  tissue,  as  will  be  mentioned  below,  dried  plants  of  the 
species  answer  fairly  well  for  a  cursory  examination  of  the  leaf  anatomy. 

Young  as  well  as  older  leaves  were  studied.  The  latter  measured 
about  2.8  by  8.3  mm.  in  size  and  the  former  was  approximately  half 
as  large. 

The  mature  leaves  assume  a  fairly  upright  position  on  the  shrub, 
of  which  the  branches  are  upright  also,  but  the  younger  leaves  are 
more  or  less  horizontally  placed  (plate  19b). 

A  cross-section  of  an  old  leaf  shows  that  the  structure  is  isosym- 
metrical.1  Colorless  parenchyma  make  up  the  middle  of  the  leaf, 
and  the  chlorenchyma  is  composed  of  about  two  layers  of  relatively 
long  palisades  on  either  side. 

Sclerenchyma  occurs  in  connection  with  the  fibro-vascular  bundles, 
and  in  that  position  forms  a  prominent  element  of  the  leaf-tissues. 

Crystals  are  fairly  abundant  in  the  outer  layers  of  the  mesophyll  and 
in  the  inner  ends  of  the  innermost  layer  of  palisade  cells. 

In  the  young  leaf  the  epidermis  is  fairly  light,  and  the  outer  wall, 
although  thin,  is,  however,  cuticularized.  Cover-hairs  with  heavy 
walls  are  present  in  the  young  leaf.  The  epidermis  of  the  old  leaf 
is  heavy,  in  part  because  of  the  much  thickened  outer  wall.  Tri- 
chomes  are  wanting. 

Stomata  occur  on  both  surfaces  of  the  leaves  (fig.  8c).  They  are 
somewhat  sunken,  especially  in  the  older  leaves,  and  are  provided 
with  a  canal  which  is  nearly  closed  by  the  permanent  constriction  of  the 
distal  portion. 

The  leaf-surface  is  slightly  roughened,  possibly  owing  to  the  per¬ 
sistence  of  the  very  bases  of  the  trichomes,  which  fall  away  as  the  leaf 
matures,  and  it  is  noticeable  that  the  surface  is  free  of  any  covering,  as 
of  wax. 

Antizoma  capensis. 

The  twiner  Antizoma  capensis  was  collected  at  White  Hills,  and 
only  dried  material  was  available  for  examination. 

The  oval  leaves,  about  10  cm.  in  length,  have  the  appearance  of 
having  unlike  dorsal  and  ventral  sides,  although,  as  will  appear 
directly,  the  symmetry  is  really  dorsi-ventral. 

The  entire  mesophyll  and  the  chlorenchyma  consist  of  relatively 
short  palisades,  a  condition  not  common  in  the  Menispermaceae, 
according  to  Solereder,  who  reports  it  for  Cocculus  leceba  only.2  Appar¬ 
ently  intercellular  spaces  are  abundant,  and  of  a  consequence  the  cells 
are  held  but  loosely  together,  from  the  fact  that  when  the  dried  leaves 
are  prepared  by  treating  with  hot  water  and  by  placing  subsequently 
in  50  per  cent  alcohol  with  a  small  amount  of  glycerine,  and  are  then 


1  A  term  here  used  to  denote  balanced  structure  in  leaves. 

2  Systematische  Anatomie  der  Dicotyledonen.  Stuttgart,  1899,  p.  45. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


83 


sectioned  by  hand,  the  entire  mesophyll  falls  apart,  and  largely  as 
separate  cells. 

The  epidermis  has  a  relatively  heavy  outer  wall  which  is  roughened 
into  squat  papilla-like  projections.  The  lateral  and  inner  walls  are 
not  appreciably  thickened  and  apparently  do  not  become  mucilagenous. 
If  present,  a  waxy  coating  of  the  epidermis  is  not  heavy,  and  may  be 
wanting. 

Stomata  occur  on  both  leaf-surfaces  and  are  not  sunken  below  the 
general  level  of  the  leaf -surf  ace  (fig.  8d). 

GaLENIA  AFRICAN  a. 

The  structure  of  the  leaves  was  examined  in  material  which  was 
collected  at  Matjesfontein  and  put  into  a  dilute  solution  of  formaline 
while  still  fresh. 

The  leaves  of  Galenia  africana  vary  from  about  10  to  about  20  mm. 
in  length  and  are  1.5  mm.,  or  less,  in  diameter,  and  are  relatively  thin. 

A  cross-section  of  the  leaf  presents  a  striking  appearance.  So 
many  of  the  epidermal  cells  are  developed  into  vesicles  that  cursory 
inspection  suggests  the  entire  epidermis  to  be  composed  of  these  cells. 
Such  spherical  epidermal  cells,  midway  from  side  to  side  of  the  leaf, 
measure  about  0.015  mm.  as  opposed  to  about  0.0012  mm.,  the  diam¬ 
eter  of  a  more  common  type,  which  have  not  developed  in  this  manner 
(fig.  8e).  It  should  be  remarked,  however,  that  in  other  species  of 
the  Ficoidese  the  vesicular  epidermals  get  to  be  the  size  of  peas  (“Erb- 
sengrosse”)>  according  to  Solereder.1 

The  epidermal  cells  of  the  usual  type  are  with  thin  outer  walls. 
The  outer  walls  of  this  form,  however,  as  well  as  the  vesicular  epi¬ 
dermal  cells,  are  possibly  thinly  cuticularized,  as  would  be  indicated 
by  the  reaction  to  chloroiodide  of  zinc,  which  stains  the  inner  portion 
of  such  walls  blue- violet  and  leaves  the  outer  portion  golden.  A 
delicate  covering  of  wax  may  be  the  final  protection  of  the  epidermal 
cells  of  both  forms. 

Two-armed  trichomes,  “ cover7 ’  trichomes,  are  fairly  abundant. 

The  stomata  are  on  a  level  with  the  small  epidermal  cells  and  are 
well  protected  by  the  large  spherical  cells  which  arch  above  them. 

The  chlorenchyma  consists  of  palisades,  two  or  three  cells  in  depth, 
which  are  relatively  short,  but  are  alike  on  the  two  sides  of  the  leaf. 
In  the  innermost  portion  of  the  leaf  and  in  association  with  the  con¬ 
ductive  tissue  are  large  cuboid  cells  which  do  not  contain  chlorophyll 
and  are  probably  to  be  regarded  as  functioning  as  water  reservoirs. 

Large  spherical  aggregates  of  crystals,  calcium  oxalate,  occur  in 
cuboid  cells  in  the  chlorenchyma,  and  especially  in  the  innermost  layer. 

Cadaba  juncea. 

Specimens  of  Cadaba  juncea  were  collected  near  Beaufort  West 
in  winter,  when  the  rudimentary  leaves  which  appear  during  the  warm 


1  L.  e.,  p.  469. 


84 


FEATURES  OF  THE  VEGETATION  OF  THE 


seasons  were  absent.  The  short  branches  with  pointed  ends,  rush-like 
in  appearance,  are  chlorophyll-bearing  and  carry  on  the  leaf  functions. 
The  structure  of  such  branches  only  was  examined. 

A  cross-section  of  a  chlorophyll-bearing  branch  shows  a  central 
wroody  portion  with  prominent  medullary  rays,  and  a  cortex  with  well- 
differentiated  outer  and  inner  portion.  A  heavy  epidermis  is  present, 
and  stomata  of  which  the  guard-cells  are  parallel  to  the  long  axis  of 
the  branch. 

Wood  fibers  are  abundant  in  connection  with  the  woody  tissues  and 
also  occur  in  groups  in  the  chlorenchyma  as  well. 

The  inner  portion  of  the  cortex  is  for  the  most  part  composed  of 
cuboid  cells  well  filled  with  starch.  Toward  the  inner  edge  of  the 
latter  are  separate  groups  of  heavy- walled  cells,  long  and  with  lumen, 
and  along  the  outer  portion,  immediately  within  the  chlorenchyma, 
are  scattered  tracheids  (fig.  8f). 

The  chlorenchyma  consists  of  greatly  elongated,  and  hence  rela¬ 
tively  narrow,  palisades  in  the  inner  half,  of  which  the  contents  are 
noticeably  more  abundant  than  in  the  outer  portion.  Tracheids  occur 
occasionally  in  the  chlorenchyma,  sometimes  being  in  contact  with  the 
epidermis,  and  sometimes  being  wholly  surrounded  by  the  cells  of  this 
tissue  (fig.  8g).  The  contents  of  the  idioblasts  take  such  stains  and 
reagents  as  were  used  differently  from  the  neighboring  cells,  and  they 
may  contain  myrosin,  although  myrosin  cells  are  not  reported  by 
Solereder  as  having  been  found  in  Cadaba.1 

The  epidermal  cells  are  much  elongated  radially,  and  the  outer 
as  well  as  the  lateral  walls  are  colored  orange  by  chloroiodide  zinc,  the 
inner  wall  only  showing  the  reaction  for  cellulose. 

The  stomata  are  deeply  placed,  owing  to  the  excessively  heavy  outer 
epidermal  wall.  The  outer  stomatal  canal,  of  a  consequence,  is 
prominently  developed.  The  ridges  of  the  vestibule  form  a  compara¬ 
tively  small  opening  to  the  vestibule,  so  that  between  the  long  canal, 
whose  mouth  is  about  0.0026  mm.  in  diameter,  and  the  vestibule,  the 
entrance  to  which  is  even  smaller,  the  stomata  finds  marked  protection 
against  excessive  changes  in  the  moisture  content,  more  especially, 
of  the  enveloping  atmosphere. 

Both  the  chlorenchyma  and  the  tissue  just  within  contain  crystals 
of  various  size  and  form,  some  prismatic,  others  oblong  or  square,  and 
others  six-angled.  Whether  crystals  of  calcium  sulphate  are  to  be 
found  in  the  epidermal  cells  or  in  the  chlorenchyma  was  not  deter¬ 
mined.  The  treatment  of  the  material  necessary  for  the  structural 
studies  would  dissolve  gypsum  crystals,  making  their  detection  im¬ 
possible.  Such  might  be  expected  to  be  present,  however,  as  it  is 
known  to  be  found  in  Capparis  jamaicensis  and  Qther  species  of  that 
genus.  The  crystals  are  mainly  calcium  oxalate. 


1  L.  c.,  p.  84. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


85 


GREWIA  CANA. 

The  shrub  Grewia  cana  was  one  of  the  species  used  in  field  studies 
on  the  foliar  transpiring  power  of  perennials.  It  was  collected  at 
Beaufort  West  and  only  herbarium  material  was  examined  in  the 
structural  studies. 

The  leaves  of  Grewia  are  about  11  by  21  mm.  in  size  and  are  fairly 
thin.  They  have  well-marked  upper  and  lower  sides  (plate  7b). 

An  examination  of  the  leaf-structure  shows  that  the  meosphyll  is 
of  palisade  cells  throughout,  although  the  cells  of  the  ventral  side  are 
somewhat  shorter  and  more  stout  than  those  of  the  side  opposite. 
The  difference,  however,  is  not  always  so  much  as  that  indicated  by 
the  figure  (fig.  8h).  Intercellular  spaces  are  especially  prominent 
on  the  ventral  side. 

Supporting  tissue,  lignified  fibers,  occurs  on  either  side,  dorsal  and 
ventral,  of  the  fibro- vascular  bundles,  but  it  is  not  an  especially  prom¬ 
inent  feature. 

The  epidermis  of  the  dorsal  side  is  relatively  heavy  with  heavy 
outer  wall.  The  contents  of  the  epidermal  cells  stains  violet  with 
chloroiodide  of  zinc,  indicating  the  possible  presence  of  mucilage. 

The  epidermal  cells  of  the  ventral  side  are  somewhat  smaller  and 
the  outer  wall  not  so  heavy  as  in  the  case  of  the  dorsal  epidermal  cells. 

The  stomata,  which  do  not  appear  to  be  abundant  as  in  many  species, 
are  confined  to  the  ventral  side.  The  guard-cells  are  raised  somewhat 
above  the  general  level  of  the  leaf-surface  (fig.  8i),  as  commonly  is 
the  case  where  the  leaf  is  provided  with  a  permanent  covering  of 
trichomes. 

Stellate  and  appressed  trichomes  occur  thickly  on  the  ventral  side 
and  constitute  an  almost  continuous  cover.  The  rays  of  the  tri¬ 
chomes  appear  always  to  be  unicellular  and  never  to  consist  of  a  chain 
of  cells,  as  Solereder  reports  has  been  found  in  some  species.1 

Rhus  viminalis. 

The  leaves  of  Rhus  viminalis  which  were  examined  were  from  a  tree 
growing  at  Matjesfontein  which  had  been  used  in  the  studies  on  the 
foliar  transpiring  power.  Dried  material  only  was  available  for  the 
anatomical  work. 

The  leaves  of  this  species  of  Rhus  have  leaflets  85  mm.,  more  or  less, 
in  length,  and  about  8  mm.  in  width.  The  slender  leaflets  which  hang 
vertically  and  the  drooping  habit  of  many  of  the  branches  of  the  tree 
give  the  appearance  of  species  of  Salix  (plate  20a). 

A  cross-section  of  the  leaflet  shows  marked  dorsi-ventrality  (fig.  8j)  . 
Greatly  elongated  palisade  cells  reach  from  the  dorsal  epidermis  to 
about  the  middle  of  the  leaf.  The  palisade  tissue  is  continued  some¬ 
what  further  by  somewhat  shorter  palisades.  On  the  ventral  side, 


1  L.  c.,  p.  178. 


86 


FEATURES  OF  THE  VEGETATION  OF  THE 


spongy  tissue  extends  to  a  depth  approximately  one-fourth  the  diam¬ 
eter  of  the  leaf.  Intercellular  spaces  are  largely  wanting  on  the  dorsal 
side,  but  are  plentiful  on  the  ventral  side. 

Sclerenchyma  occurs  in  connection  with  the  fibro-vascular  bundles, 
as  well  as  along  the  margins  of  the  leaf. 

Sections  across  the  conductive  tissue  show  the  presence  of  ducts  in 
the  phloem.  Whether  by  anastomosing  the  ducts  form  a  network,  as 
in  certain  species  of  Rhus,  was  not  determined. 

The  epidermis  is  not  very  heavy,  although  the  outer  wall,  especially 
of  the  dorsal  side,  is  relatively  thick. 

Stomata  are  to  be  found  only  on  the  ventral  side  and  are  not  deeply 
placed  (fig.  8k). 

Shield-shaped  secreting  trichomes  are  sparingly  present  on  both 
surfaces  of  the  old  leaves. 

Rhus  sp. 

Rhus  sp.  was  used  in  studies  on  the  foliar  transpiring  power  at  the 
Karroo  Botanical  Gardens,  Whitehills,  and  dried  leaves  of  the  plant 
so  used  were  examined  anatomically. 

It  should  be  remarked  in  passing  that  the  habitat  of  this  species  at 
Whitehills  is  the  rounded  summit  of  a  kopje,  where  it  occurs  in  associa¬ 
tion  with  Euclea  sp.  The  environment  of  the  species  is  very  much 
more  arid  than  that  of  Rhus  viminalis,  which  occurs  along  the  stream¬ 
way  at  Matjesfontein,  at  which  place  the  rainfall  is  also  greater. 

The  leaflets  vary  in  form  from  obcordate  to  oblong-linear  and  in 
size  from  about  5  to  10  mm.  in  width  and  from  about  10  to  30  mm. 
in  length. 

The  leaf-structure  of  Rhus  sp.  resembles  that  of  Rhus  viminalis 
in  the  fundamental  points,  but  it  is  very  different  in  certain  features 
respecting  the  degree  of  development  of  certain  of  the  tissues. 

As  in  the  case  of  Rhus  viminalis ,  the  structural  symmetry  is  dorsi- 
ventral.  Palisades  occur  on  the  dorsal  side  and  spongy  chlorenchyma 
on  the  ventral  side.  Stomata  occur  on  the  ventral  side  only  and  are 
not  sunken  deeply,  but  are  about  on  the  general  level  of  the  surface 
of  the  leaf.  Sclerenchyma  occurs  along  with  the  conductive  tissue, 
and  ducts  are  present.  When  examined  more  closely,  however,  each  of 
these  tissues  has  a  character  different  from  that  of  the  corresponding 
tissue  in  the  other  species. 

The  outer  epidermal  wall  on  the  dorsal  surface  is  not  especially 
thick,  but  it  is  at  least  in  part  covered  by  a  heavy  mass  of  resinous 
substance  apparently  secreted  by  trichomes  (fig.  8l).  This  feature 
is  quite  as  well  marked  on  the  ventral  side,  where  irregularities  in  the 
surface  of  the  leaf  are  quite  filled  by  the  resin,  a  feature  wanting  in 
the  leaves  of  Rhus  viminalis  examined. 

The  chlorenchyma  of  Rhus  sp.  is  in  part  palisades  and  in  part 
composed  of  cuboid  cells,  both  with  small  intercellular  spaces,  and  of 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


87 


spongy  parenchyma.  The  outer  layer  of  palisades  is  about  0.07 
mm.  in  length  as  contrasted  to  the  corresponding  palisades  of  Rhus 
viminalis,  which  measure  about  0.4  mm.,  and  make  up  about  one-fifth 
of  the  chlorenchyma.  From  this  it  will  appear  that  the  relative 
amount  of  palisade  tissue  in  Rhus  sp.  is  much  less  than  that  of  the  other 
species. 

Supporting  tissue  is  associated  with  the  fibro-vascular  bundles,  but 
it  is  not  so  pronounced  a  feature  as  in  Rhus  viminalis.  On  the  other 
hand,  the  ducts  which  occur  in  the  phloem  of  the  fibro-vascular  bundles 
are  very  much  larger  than  in  Rhus  viminalis. 

Gymnosporia  buxifolia. 

Gymnosporia  buxifolia  was  used  at  Beaufort  West  in  connection 
with  studies  on  the  foliar  transpiring  power  of  some  of  the  Karroo 
perennials.  This  species,  according  to  Bews,1  is  somewhat  variable 
and  is  widespread  all  over  South  Africa.  It  is  very  spiny  in  the  drier 
situations.  The  general  habit  of  the  species  and  the  character  of  the 
leaves  are  shown  in  the  illustrations  (plate  7c  and  plate  7a).  It  will 
be  seen  from  the  figures  that,  although  not  large,  the  leaves  are  abun¬ 
dant  and  their  positions  bears  apparently  no  especial  relation  to  the 
incident  light  rays. 

A  cross-section  or  a  longitudinal  section  of  a  typical  mature  leaf 
shows  several  striking  features.  The  epidermis,  with  heavy  outer 
wall,  nearly  obliterating  the  lumen,  is  underlaid  by  one  or  two  layers 
of  cuboid  cells,  beneath  which  is  the  chlorenchyma  proper.  Schleren- 
chyma  occurs  in  association  with  the  conductive  tissue.  Stomata 
are  numerous  and  are  to  be  found  on  both  surfaces  of  the  leaf.  They 
are  level  with  the  outer  epidermal  wall,  or  even  project  somewhat. 
There  is  little  or  no  external  vestibule  and  the  substomal  chamber  is 
small  (fig.  8m). 

The  walls  of  the  hypoderm  and  the  inner  and  lateral  walls  of  the 
epidermis  are  light.  Although  in  older  leaves  the  hypodermal  cells 
are  apparently  without  contents,  for  the  most  part  there  are  occa¬ 
sional  cells  of  this  tissue  which  contain  crystal  aggregates  which  nearly 
fill  them.  Similar  secretion  cells,  with  calcium  oxalate  crystals  of 
the  same  appearance,  occur  in  the  chlorenchyma  as  well.  In  the 
middle  of  the  leaf  the  chlorophyll-bearing  cells  are  cuboid,  or  may 
even  have  the  long  axis  parallel  to  the  leaf-surface,  but  for  the  most 
part  the  chlorenchyma  consists  of  short  palisades.  Intercellular 
spaces  are  apparently  but  poorly  developed. 

CUSSONIA  SPICATA. 

The  material,  which  was  studied  in  the  dried  condition,  was  col¬ 
lected  on  the  upper  north  face  of  a  fiat-topped  and  low  mountain 

1  The  Southeast  African  flora:  Its  origin,  migrations,  and  evolutionary  tendencies.  Ann. 
Bot.,  36,  216,  1922. 


88 


FEATURES  OF  THE  VEGETATION  OF  THE 


near  Beaufort  West,  where  it  occurs  sparingly.  The  large  palmate 
leaves  of  the  plant,  which  are  borne  only  at  the  summit  of  the  branches, 
give  a  subtropical  appearance,  somewhat  out  of  keeping  with  the  re¬ 
mainder  of  the  vegetation  of  the  vicinity. 

The  leaves  of  Cussonia  are  about  32  by  160  mm.  and  are  of  a  leathery 
texture.  They  are  distinctly  bifacial  in  appearance. 

The  most  striking  single  characteristic  of  the  structure  of  the  leaf 
is  the  presence,  on  the  dorsal  side  only,  of  a  hypoderm  consisting 
of  three  or  four  layers  of  cells.  Between  the  veins  the  cells  of  this 
tissue  are  cuboid,  but  opposite  the  fibro-vascular  bundles  they  are 
somewhat  elongated.  On  both  sides  of  the  leaf,  and  in  relation  to  the 
conductive  tissue,  thus  dorsal  and  ventral  as  well,  the  hypodermal 
cells  constitute  the  entire  issue  to  the  epidermis.  This  condition  is 
at  least  true  of  the  larger  “  veins. ”  It  will  be  seen,  therefore,  that  the 
supporting  tissue  is  unusually  well  developed  in  the  species  and  that 
to  a  degree  the  chlorenchyma  is  separated  into  separate  masses. 
The  general  character  of  the  hypoderm  is  given  in  figure  13a. 

The  true  epidermis  of  the  dorsal  side  of  the  leaf  is  not  especially 
heavy,  although  the  outer  wall  is  very  thick  and  has  a  much  thickened 
cuticle.  That  of  the  ventral  side  is  also  heavy  and  projects  into  short 
papillae,  giving  to  the  surface,  when  viewed  under  a  microscope,  a 
somewhat  roughened  appearance.  The  dorsal  leaf-surface,  on  the 
other  hand,  is  fairly  smooth.  The  epidermal  cells  of  the  ventral  side 
have  granular  contents,  the  nature  of  which  was  not  investigated, 
while  the  dorsal  epidermal  cells  appear  to  be  entirely  empty. 


Fig.  9. — a,  Cussonia  spicata,  detail  of  epidermis,  showing  stomata,  and  cuboid  chlorenchyma. 
Ventral  surface.  X  300. 

b,  Cussoma  spicata,  dorsal  surface  of  leaf,  showing  heavy  outer  epidermal  wall  and 

hypoderma,  several  cells  in  thickness,  with  palisades  within.  X300. 

c,  Fragment  of  leaf  of  Cotyledon  paniculata,  prepared  from  material  grown  at  the  Coastal 

Laboratory,  showing  the  cuboid  chlorenchyma  and  delicate  epidermis.  X70. 

d,  Cross-section  of  leaf  of  Stachys  sp.,  showing  the  confused  mass  of  trichomes  on  both 

surfaces  and  the  prominently  developed  palisades  on  the  dorsal  side,  with  spongy 
parenchyma  on  the  ventral  side  and  stomata  with  guard-cells  which  project 
slightly.  X300. 

e,  Aptosimum  indivisum,  cross-section  of  leaf,  showing  heavily  developed  outer  epi¬ 

dermal  wall,  short  palisades,  and  superficially  placed  stomata.  X300. 

/,  Carissa  ferox,  fragment  of  ventral  side  of  leaf  showing  heavy  outer  epidermal  walls, 
with  superficially  placed  stomata,  having  two  subsidiary  cells.  The  palisade-like 
character  of  the  outer  chlorenchyma  of  the  ventral  side  is  indicated.  There  prob¬ 
ably  are  better  developed  intercellular  spaces,  however,  than  are  shown  in  the 
sketch.  X325. 

g,  Asclepias  filiformis  (?),  semi-diagrammatic  cross-section  of  leaf,  showing  channelling 

of  leaf  and  disposition  of  main  tissues.  The  two  separate  masses  of  chlorenchyma 
are  indicated.  For  further  explanation,  see  text.  X70. 

h,  Asclepias  filiformis  (?),  detail  from  dorsal  side  of  leaf,  showing  the  heavy  outer  epi¬ 

dermal  wall  and  character  of  stomata.  X300. 

i,  Euclea  undulata,  fragment  of  cross-section  of  ventral  side  of  leaf,  showing  the  super¬ 

ficial  placing  of  the  stomata  and  character  of  the  epidermis.  X300. 

j,  Euclea  undulata,  portion  of  cross-section  of  leaf  to  show  the  heavy  epidermis,  prom- 

nent  development  of  palisade  cells,  and  sclerenchyma.  X300. 

k,  Eriocephalus  sp.,  cross-section  of  leaf,  with  the  most  prominent  tissues  outlined: 

e,  epidermis;  A,  conductive  tissue;  p,  chlorenchyma.  X225. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


89 


/ 


90 


FEATURES  OF  THE  VEGETATION  OF  THE 


Stomata  occur  on  the  ventral  side  only  and  are  not  sunken,  -but  are 
about  level  with  the  leaf-surface  (fig.  9b). 

Secreting  ducts  occur  in  relation  to  the  phloem  of  the  fibro- vascular 
bundles  and  are  not  found  in  the  chlorenchyma. 

The  chlorophyll-bearing  tissue  is  in  part  of  poorly  developed  pali¬ 
sades  and  in  part  of  spongy  parenchyma.  The  former  are  about 
four  cell  layers  deep  and  lie  immediately  beneath  the  hypoderm,  and 
the  latter  are  on  the  ventral  side.  Intercellular  spaces  are  more  abun¬ 
dant  in  the  latter  than  in  the  former  type  of  chlorenchyma,  although 
apparently  not  a  pronounced  feature  of  either. 

Bauhinia  marlothii. 

Bauhinia  marlothii  was  collected  in  the  habitat  of  Welwitschia 
mirabilis  in  the  Namib.  Scattered  along  the  same  shallow  depression 
in  the  desert  plain  were  Asclepias  filiformis  and  Zygophyllum  stapfii 
(plate  3).  Only  dried  material  was  available  for  the  anatomical 
investigation,  which,  for  reasons  to  appear  directly,  was  not  entirely 
satisfactory. 

The  leaflets  examined  were  orbicular  in  form,  fairly  abundant,  and 
and  did  not  suggest  the  extreme  aridity  of  the  habitat.  The  following 
were  the  most  striking  points  in  the  structure  of  the  leaves.  Except 
for  the  usual  dorsi-ventral  arrangement  of  the  conductive  system,  the 
structure  of  the  two  sides  of  the  leaf  is  alike.  That  is,  palisade  cells 
similar  in  form  occur  both  on  the  ventral  and  the  dorsal  side,  with 
cuboid  parenchyma  between.  Stomata  occur  on  both  surfaces  of  the 
leaf.  Trichomes  were  not  found  and  sclerenchymatous  tissue  of  any 
kind  is  wanting.  It  is  owing  to  the  last-named  circumstance  that, 
as  suggested  above,  dried  material  is  not  suitable  for  anatomical 
study  of  the  leaf. 

In  the  young  leaf  the  epidermal  cells  are  relatively  large,  and  in  age 
these  cells  enlarge  greatly  and  become  almost  vesicular.  The  outer 
walls,  which  are  relatively  thin,  are  therefore  markedly  convex.  The 
stomata  in  the  old  leaf  are  superficially  placed,  and,  although  fairly 
numerous,  appear,  as  compared  to  those  of  many  species,  to  be  of 
small  size. 

SUTHERLANDIA  FRUTESCENS. 

The  material  of  Sutherlandia  frutescens  used  in  the  anatomical  study 
was  collected  near  a  streamway  about  6  miles  east  of  Beaufort  West, 
where  it  occurs  together  with  Senecio  longifolius,  Mesembryanthemum 
unidens,  and  other  perennials.  As  stated  elsewhere  in  this  paper,  the 
habitat  is  apparently  less  arid  than  Beaufort  West,  holding  also 
somewhat  different  relations  to  the  mountains. 

Although  the  material  at  hand  was  not  very  satisfactory  for  ana¬ 
tomical  study,  it  was  possible  to  make  out  the  following  general  struc- 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


91 


tural  features  of  the  leaflets.  The  mesophyll  is  of  palisade  cells 
throughout  with  intercellular  spaces  apparently  well  developed. 
Stomata  occur  on  both  surfaces  and,  at  least  in  the  young  leaves,  there 
is  a  sparse  covering  of  non-secreting  trichomes.  The  epidermis,  both 
of  the  dorsal  and  the  ventral  sides,  is  fairly  heavy.  Supporting  tissue 
of  whatever  kind,  that  is  to  say  sclerenchyma,  is  absent. 

Cotyledon  paniculata. 

Cotyledon  paniculata ,  which  is  one  of  the  most  striking  species  with 
water-storage  capacity  in  southern  Africa,  has  a  stout  stem  and  heavy, 
short  branches  (plate  18b).  The  leaves  are  formed  in  early  autumn 
and  persist  through  the  winter  months  and  into  the  spring,  until  the 
advent  of  warm  and  dry  weather.  Then  they  fall  and  the  summer 
is  passed  with  only  such  carbon-dioxide  assimilation  as  may  be  possi¬ 
ble  on  the  part  of  the  green  branches. 

Studies  on  the  transpiring  power  of  the  leaves  of  this  species  were 
carried  out  at  Matjesfontein. 

The  plants  used  in  the  anatomical  study  were  collected  at  Matjes¬ 
fontein  and  came  into  leaf,  following  irrigation,  in  early  summer. 
Leaves  from  small  plants  were  studied  in  the  fresh  condition. 

The  leaves  used  measured  about  80  by  35  mm.  and  had  a  thickness 
of  about  2.5  mm. 

The  leaf  has  the  general  structure  of  leaves  of  succulents.  It  is 
mechanically  weak.  The  epidermis  is  light  and  is  easily  stripped  in 
ribbons  from  either  surface.  The  chlorenchyma  is  of  cuboid  paren¬ 
chyma  only  and  has  abundant  intercellular  spaces.  Even  in  the 
middle  of  the  leaf  chlorophyll  is  present  in  abundance.  There  is  no 
supporting  tissue  and  the  conductive  system  appears  to  be  meager. 
Glandular  trichomes  occur  on  both  surfaces  (fig.  9c).  The  meso¬ 
phyll  cells  are  highly  mucilaginous  and,  although  quite  turgid  when 
placed  in  water,  they  nevertheless  increase  in  thickness  appreciably 
and  the  leaf  usually  exhibits  curving,  but  apparently  only  in  a  certain 
direction.  Whether,  however,  the  dorsal  surface  uniformly  becomes 
concave  under  such  conditions,  as  in  the  young  plant,  was  not  learned. 

The  outer  epidermal  wall  of  both  leaf-surfaces  in  the  material 
used  was  found  to  be  thin,  and  of  a  consequence  the  stomata,  which 
occur  on  both  sides  of  the  leaf,  are  not  sunken.  The  stomata  are  rela¬ 
tively  not  abundant.  For  example,  it  was  found  that  per  unit  area 
there  were  approximately  twice  as  many  stomata  on  the  ventral  side  of 
a  sunflower  leaf  as  was  counted  in  Cotyledon.  Whether,  however, 
the  same  would  be  true  for  a  corresponding  specimen  growing  on  the 
veld  at  Matjesfontein,  I  have  no  means  of  finding  out.  The  findings 
are  opposed  to  the  usual  experience  that  for  the  same  area  the  number 
of  stomata  is  greater  in  xerophytes  than  in  mesophytes. 


92 


FEATURES  OF  THE  VEGETATION  OF  THE 


Stachys  sp. 

The  species  of  Stachys  studied,  of  which  only  dried  material  was 
available,  was  collected  on  the  south  side  of  a  canyon  about  6  miles 
east  of  Beaufort  West,  at  a  place  where  the  vegetation  was  relatively 
abundant,  both  as  to  the  number  of  species  and  individuals.  Among 
other  forms  the  following  perennials  were  found  in  the  vicinity: 
Cussonia  spicata,  Cadaba  juncea,  Eriocephalus  sp.,  Euclea  undulata , 
and  Pteronia  incana. 

The  leaves  measured  15  by  20  mm.  or  less  in  size  and  are  markedly 
bifacial  and  otherwise  have  the  general  character  of  the  family.  In 
section  they  exhibit  the  following  main  structural  features:  The  leaf 
is  relatively  thin.  The  conductive  tissue  is  well  developed  and  con¬ 
stitutes  the  main  supporting  system.  Heavy- walled  mechanical 
tissue  appears  to  be  quite  absent.  The  structural  symmetry  is 
strongly  dor  si- ventral.  Both  leaf-surfaces  are  very  heavily  clothed 
with  cover  trichomes.  The  stomata  are  confined  to  the  ventral  sur¬ 
face. 

When  examined  somewhat  in  greater  detail,  it  is  found  that  the 
epidermis  is  not  heavy  and  the  outer  wall  is  thin.  Apparently  every 
epidermal  cell,  or  by  far  the  most  of  them,  of  the  dorsal  side  gives 
rise  to  a  trichome.  The  same  is  also  true,  but  possibly  to  a  somewhat 
less  extent,  of  the  cells  of  the  ventral  epidermis  (fig.  9d).  The  tri¬ 
chomes  are  so  abundant  as  to  form  a  felted  covering  to  the  leaf.  This 
remark  refers  especially  to  the  young  leaf.  In  age,  possibly  because 
of  the  increase  in  size  of  the  epidermal  cells,  and  possibly  also  because 
of  the  shedding  of  some  of  the  trichomes,  the  pubescence  appears  not 
to  be  so  dense.  In  such  leaves  epidermal  cells  can  more  easily  be  made 
out  between  the  trichomes.  In  a  section  of  a  leaf  where  the  trichomes 
remain  it  is  found  that  the  felted  trichomal  mass  of  either  leaf-surface 
exceeds  the  diameter  of  the  main  part  of  the  lamina  itself. 

The  stomata  are  placed  so  that  they  not  only  are  not  depressed 
below  the  general  level  of  the  leaf,  but  on  the  contrary  the  guard- 
cells  are  noticeably  elevated.  As  will  be  commented  on  elsewhere 
in  this  paper,  this  condition  has  been  found  to  be  the  frequent  or 
possibly  the  invariable  accompaniment  of  pubescence  of  the  leaf. 


Aptosimum  indivisum. 

Aptosimum  indivisum  was  collected  on  the  northern  exposure  of  a 
dolorite  kopje  near  Beaufort  West,  where  Euphorbia  mauritanica , 
Pentzia  virgata,  and  Asparagus  striatus  also  occur.  Leaves  of  the 
rosette,  and  dried  material  only,  were  used  in  the  anatomical  study. 

The  rosette  leaves  are  about  6  by  20  mm.  in  size  and  have  the  appear¬ 
ance  of  being  bifacial.  They  are  clustered  thickly  on  the  shortened 
stem  and  do  not  appear  to  have  especial  orientation. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


93 


In  cross-section  the  leaf  is  found  to  have  a  very  simple  structure. 
The  mesophyll  is  of  short  palisades  throughout.  Stomata  occur  on 
both  surfaces.  Sclerenchyma  is  but  little  developed,  only  accom¬ 
panying  the  fibro-vascular  bundles.  The  outer  epidermal  wall  is  of 
great  relative  thickness  and  apparently  constitutes  the  leading  mechan¬ 
ical  element  of  the  leaf. 

In  the  material  studied  trichomes  were  not  present,  so  that  it  was 
somewhat  surprising  to  find  the  guard-cells  slightly  elevated  above  the 
epidermal  cells  proper,  although,  owing  to  the  heavy  outer  epidermal 
wall,  they  are  slightly  below  the  general  level  of  the  leaf  (fig.  9e). 

The  ridges  of  the  outer  vestibule  are  prominently  developed,  in 
fact  more  so  than  is  indicated  by  the  figure. 

The  outer  surface  of  the  epidermal  wall  is  slightly  ribbed,  giving  the 
leaf-surface  fine  striations.  The  cuticle  is  light. 

Carissa  ferox. 

The  material  of  Carissa  ferox  which  was  used  in  the  anatomical 
study  was  from  quadrat  No.  1,  on  the  southern  slope  of  a  dolorite  kopje 
near  Beaufort  West.  It  was  associated  with  Euphorbia  mauritanica , 
Grewia  cana,  and  other  perennials  (plate  8c) .  The  species  is  intensely 
spiniferous.  The  smaller  branches  and  spines,  at  least,  are  chloro¬ 
phyll-bearing.  The  oval  leaves  are  about  10  by  25  mm.  in  size,  are 
relatively  thick,  and  leathery  in  texture. 

Dried  material  only  was  available  for  the  study  of  the  leaf-struc¬ 
ture.  A  general  view  of  the  anatomy  shows  the  following  important 
features:  The  structure  is  dorsi-ventral,  although  the  tendency  of 
the  chlorenchyma  of  the  ventral  side  toward  palisade  form  may  be 
noted.  Throughout  the  central  portion  of  the  leaf  and  only  in  part 
directly  connected  with  the  fibro-vascular  bundles  is  to  be  found  the 
lactiferous  tissue.  Sclerenchyma  is  present  in  connection  with  the 
conductive  tissue,  but  apparently  only  in  association  with  the  xylem 
elements.  The  epidermis,  especially  of  the  dorsal  side,  is  heavy, 
because  of  the  extremely  thick  outer  wall.  Trichomes  are  sparingly 
present  on  both  leaf-surfaces,  and  where  they  take  their  origin  the 
lumen  of  the  epidermal  cell  is  somewhat  larger  than  that  of  the  con¬ 
tiguous  epidermal  cells,  from  the  fact  that  the  inner  secondary  thick¬ 
ening  of  the  outer  wall  in  such  cells  fails.  This  condition  results  in  a 
circular  and  narrow  band  at  the  base  of  each  trichome  where  the 
“protection”  against  possible  water-loss,  such  as  that  afforded  the 
outer  leaf  cells  by  the  thicker  neighboring  outer  epidermal  wall,  is 
to  a  certain  extent  wanting.  The  stomata  occur  only  on  the  ventral 
side.  They  are  about  flush  with  the  general  level  of  the  leaf-surface. 
There  are  two  subsidiary  cells  (fig.  9f). 


94 


FEATURES  OF  THE  VEGETATION  OF  THE 


ASCLEPIAS  FILIFORMI8  (?). 

The  material  of  Asclepias  filiformis  (?)  which  was  used  in  the  ana¬ 
tomical  studies  was  collected  in  the  habitat  of  Welwitschia  mirabilis  in 
the  Namib.  Owing  to  the  extreme  aridity  of  the  habitat,  more  than 
usual  interest  attaches  to  the  peculiarities  of  the  species  which  can 
survive  its  rigors.  Among  such  are,  in  addition  to  those  above  men¬ 
tioned,  Arthrcerua  marlothii  and  Zygophyllum  stapfii.  Although  ex¬ 
posed  to  a  common  environment,  these  species  have  nevertheless 
developed  in  different  directions,  all  being  perennials.  The  general 
character  of  the  species  need  only  be  mentioned  in  this  place.  That  of 
Welwitschia  is  well  known.  This  species  has  a  well-developed  tap-root 
with  large  crown  which  may  act  in  the  capacity  of  storage  organ  for 
water.  Asclepias  has  apparently  no  storage  capacity,  while  the 
branches  of  Arthrcerua  are  somewhat  fleshy  (plates  4a  and  5b),  and 
the  leaves  only  of  Zygophyllum  are  succulent  (plates  2c  and  3c). 

Dried  material  only  was  used  in  the  anatomical  study.  The  leaves 
are  strap-shaped  but  exceedingly  long.  Although  both  old  and  young 
leaves  were  examined,  this  report  refers  mainly  to  a  leaf  measuring 
2.5  by  150  mm.  A  cross-section  of  such  a  leaf  shows  three  main 
regions,  namely,  a  central  region  dominated  by  the  midrib,  and  two 
lateral  regions  wholly  separated  by  the  midrib.  The  latter  consist 
of  chlorenchyma  in  the  midst  of  which  are  several  small  fibro-vascular 
bundles.  The  latter  are  not  indicated  in  the  general  sketch  of  the 
tissues  of  the  leaf  (fig.  9g).  As  the  figure  shows,  the  leaf  is  strongly 
channeled  with  two  grooves  on  the  ventral  side  and  one  opposite  their 
intersection  on  the  dorsal  side. 

The  tissues  of  the  midrib  region  consist  of  the  fibro-vascular  bundle, 
of  which  the  vascular  part  is  very  pronounced,  being  composed  in 
outline  of  the  V-shaped  mass  of  the  figure  referred  to,  and  two  masses 
of  cuboid  cells  toward  the  leaf-surfaces.  Of  these,  that  dorsal  to  the 
conductive  tissue  is  wholly  of  thin  walls  and  contains  a  half  dozen 
or  more  lactiferous  ducts;  but  on  the  opposite  side  the  corresponding 
cells  are  with  heavy  walls  near  the  leaf-surface  and  with  light  walls 
nearer  the  fibro-vascular  bundle.  None  of  these  cells  bears  chloro¬ 
phyll.  Their  walls  are  not  lignified,  but  with  zinc-chloroiodide  give 
the  reaction  for  cellulose.  Intercellular  spaces  are  present  in  the 
cuboid  tissue  of  either  side,  except  only  for  the  most  part  in  that  with 
heavy  walls,  where  they  appear  to  be  wanting. 

As  indicated  above,  the  tissue  containing  chlorophyll  consists  of 
two  ribbon-like  masses  running  lengthwise  of  the  leaf  and  wholly 
separated  from  each  other.  On  the  dorsal  side  the  chlorenchyma  is  of 
palisades  and  on  the  ventral  side  it  is  of  spongy  parenchyma. 

Stomata  occur  sparingly  on  both  leaf-surfaces,  as  indicated  dia- 
grammatically  in  figure  9g.  They  are  placed  superficially  (see  figure 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


95 


9h),  which  is  somewhat  to  one  side  of  the  median  line  of  the  stoma, 
and  do  not  have  pronounced  outer  ridges  or  vestibule. 

The  epidermis  has  a  heavy  outer  wall  opposite  the  chlorenchyma 
and  is  overlaid  with  a  coating  of  wax,  not  only  in  the  portion  asso¬ 
ciated  with  the  chlorophyll-bearing  cells,  but  opposite  the  midrib 
as  well. 

The  point  should  be  brought  out  in  passing  that  there  is  no  heavy- 
walled  supporting  tissue  in  the  leaves.  In  the  slender  stems,  however, 
such  is  to  be  found.  This  is  in  the  primary  cortex,  where  there  is  a 
ring  of  hard  bast  composed  of  bundles  separated  from  each  other  by  a 
radial  layer  of  parenchyma.  It  is  largely  to  this  mechanical  tissue 
that  the  rigidity  of  the  stem  is  due. 

Euclea  undulata. 

The  material  used  in  the  anatomical  study  was  collected  about 
6  miles  east  of  Beaufort  West.  In  the  same  habitat  were  numerous 
other  perennials,  among  which  were  Cussonia  spicata,  Cadaba  juncea, 
Eriocephalus  sp.  and  Pteronia  incana,  of  which  the  leaf-structure  was 
studied,  and  others.  Observations  on  the  foliar  transpiring  power  of 
Euclea  undulata  were  carried  out  at  Matjesfontein. 

Euclea  undulata  is  a  large  shrub.  The  evergreen  leaves  are  numer¬ 
ous  and  relatively  large,  measuring  about  20  by  30  mm.  or  less,  and 
are  confined  to  the  tips  of  the  branches.  Both  young  and  old  leaves 
were  examined,  with  the  following  results:  The  structure  is  dorsi- 
ventral.  Trichomes  are  abundant  on  both  leaf-surfaces.  Stomata  are 
confined  to  the  ventral  side.  There  is  marked  development  of  sup¬ 
porting  tissue. 

Trichomes  of  different  types,  some  of  which  are  glandular,  are 
especially  abundant  in  the  younger  leaf.  A  correlation  between  light 
outer  epidermal  wall  and  the  presence  of  trichomes,  and  of  a  heavy 
outer  wall  where  they  are  wanting,  was  noted. 

The  outer  wall  of  the  epidermis,  both  of  the  ventral  and  of  the 
dorsal  sides,  is  very  heavy  and  the  cuticle  well  developed.  The  inner 
epidermal  wall  also  is  heavy  (fig.  9i).  The  stomata  do  not  appear 
to  be  very  abundant  and  are  not  sunken  appreciably  below  the  general 
level  of  the  leaf. 

The  chlorenchyma  consists  of  about  one  cell-layer  of  palisades,  with 
the  balance  cuboid  cells  or  spongy  parenchyma.  In  the  latter  case 
intercellular  spaces  are  abundant. 

Sclerenchyma  occurs  both  on  the  dorsal  and  the  ventral  sides  of 
the  conductive  tissue,  and  between  them  and  the  epidermis.  That  in 
connection  with  the  midrib  is  especially  heavy  (fig.  9j).  Exterior 
to  the  strand  of  sclerenchyma  and  connecting  it  to  the  epidermis  of 
either  surface  are  masses  of  heavy-walled  cuboid  cells,  which  are 
apparently  without  contents  in  age.  This  refers  to  the  midrib. 
Apparently  in  the  smaller  fibro- vascular  systems  such  tissue  is  wanting. 


96 


FEATURES  OF  THE  VEGETATION  OF  THE 


Royena  pallens. 

The  material  of  Royena  pallens  studied  was  collected  on  the  lower 
slopes  of  a  kopje  about  2  miles  or  less  from  Beaufort  West.  The 
character  of  the  habitat  is  shown  in  plate  6b.  The  leading  species  of 
the  general  habitat  are  listed  at  page  66. 

The  leathery  leaves  are  2  mm.,  more  or  less,  in  length  and  6  mm.,  or 
less,  in  width.  They  have  distinct  upper  and  lower  faces. 

A  cross-section  of  mature  leaves  shows  the  following  leading  struc¬ 
tural  characters:  The  epidermis,  which  is  not  very  heavy,  is  com¬ 
posed  of  relatively  small  cells.  The  outer  wall  is  heavy  and  its  outer 
portion  is  cuticularized.  The  surface  of  the  cuticle  is  striated.  Cover 
trichomes  are  apparently  abundant  in  young  leaves,  but  in  old  leaves 
they  are  largely  wanting.  They  occur  on  both  surfaces.  Stomata 
are  present  on  the  ventral  side  only  and  are  not  sunken  below  the 
general  level  of  the  surface  (fig.  10g).  The  chlorenchyma  consists  of 
palisades  only,  which  are  not  alike  on  both  sides  of  the  leaf.  Those 
of  the  ventral  side  are  somewhat  less  palisade-like  and  the  intercellular 
spaces  of  this  side  are  apparently  somewhat  more  abundant.  The 
tissue  of  the  middle  part  of  the  mesophyll  is  parenchyma  with  prom¬ 
inent  intercellular  spaces.  The  walls  are  of  cellulose.  A  portion 
of  the  cells  contains  very  large  single  crystals.  Sclerenchyma  is  not 
present,  either  in  connection  with  the  fibro-vascular  bundles  or  else¬ 
where. 

Ekiocephalus  sp. 

Eriocephalus  sp.  was  observed  at  Matjesfontein,  where  the  material 
studied  was  collected.  It  constitutes  one  of  the  population  of  quadrat 

Fig.  10. — a,  Eriocephalus  sp.,  cross-section  of  lead,  to  show  the  absence  of  lumen  in  the  epi¬ 
dermal  cells  because  of  the  secondary  thickening  of  the  walls.  The  bases  of  two 
trichomes  are  shown,  and  the  pronounced  development  of  palisades  indicated. 
X300. 

h,  Euryops  lateriflorus ,  section  of  leaf  in  which  the  heavy  epidermis,  deeply  placed 
stomata,  and  short  palisades  are  indicated.  X300. 

c,  Pentzia  virgata,  cross-section  of  leaf  to  show  the  superficially  placed  stomata,  rela¬ 

tively  thin  epidermis  with  heavy  outer  wall,  secreting  trichome,  and  two-ranked 
palisade  tissue.  X300. 

d,  Pteronia  flexicaulis,  cross-section  showing  the  distribution  of  the  following  tissues: 

o,  epidermis;  fv,  conductive  tissue  with  sclerenchyma  in  circles;  p,  chlorenchyma. 
X150. 

e,  Pteronia  flexicaulis,  cross-section  showing  the  heavy  epidermis  with  greatly  thickened 

outer  wall  and  superficially  placed  stomata.  X300. 

/,  Pteronia  incana,  cross-section  of  leaf,  in  which  the  following  are  shown:  /,  duct; 
fv,  conductive  issue;  p,  chlorenchyma  with  the  epidermis  without  the  dotted  line. 
X50. 

g,  Ptergonia  incana,  cross-section  of  leaf,  showing  heavy  outer  epidermal  wall  and 

relatively  short  palisades.  X300. 

h,  Rovena  pallens,  cross-section  of  leaf  secretion  on  the  surface  of  the  epidermis,  the 

superficially  placed  stomata,  and  palisades.  X300. 

i,  Stoebe  sp.,  semi-diagrammatic  cross-section  of  leaf,  showing  the  extent  of  the  mass 

of  trichomes  on  the  ventral  side,  within  the  curving  broken  line,  the  width  of  the 
epidermis  and  the  conductive  tissue  having  sclerenchyma,  indicated  by  circles, 
on  either  side.  X60. 

j,  Stctbe  sp.,  cross-section  of  leaf,  showing  modified  dorsi-ventral  symmetry  of  struc¬ 

ture.  The  light  epidermis,  stomata  with  projecting  guard-cells,  and  trichomes  of 
the  ventral  side  are  sharply  contrasted  with  the  heavy  epidermis,  the  absence  of 
stomata  and  of  trichomes  of  the  dorsal  side.  X300. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


97 


Fig.  10. 


98 


FEATURES  OF  THE  VEGETATION  OF  THE 


No.  3,  where  it  occurs  in  association  with  Aster  filij or mis,  Cotyledon 
orbiculata,  C.  reticulata,  Pteronia  glomerata ,  Stoebe  sp.,  and  other  forms. 

The  leaves  of  this  species  are  terete  in  cross-section  and  about  6  mm. 
in  length.  They  are  borne  in  groups,  frequently  opposite,  and  usually 
on  dwarf  shoots. 

A  section  of  a  leaf  shows  a  prominent  epidermis,  a  band  of  ehloren- 
chyma,  of  palisades  without,  and  fibro-vascular  bundles  with  promi¬ 
nent  masses  of  supporting  tissue  (fig.  13k).  Cover  trichomes  are 
especially  abundant  in  young  leaves,  arising  from  nearly  every  epi¬ 
dermal  cell. 

A  most  striking  structural  feature  of  the  leaf  is  the  epidermis,  in 
which,  because  of  the  excessive  thickness  of  the  walls,  the  lumen  is 
quite  obliterated.  The  differential  staining  obtained  with  chloroiodide 
of  zinc  is  of  interest.  The  bases  of  the  trichomes  and  outer  portion 
of  the  outer  wall  become  orange,  being  cuticularized,  while  the  greatly 
thickened  walls  otherwise  become  deep  violet  with  this  reagent. 

The  stomata  are  fairly  deeply  placed  (fig.  10a),  having  the  thickness 
of  the  epidermis  between  the  guard-cells  and  the  leaf-surface.  In 
the  development  of  the  subsidiary  cells,  which  are  modified  like  the 
balance  of  the  epidermis,  there  has  been  left  no  especial  constriction 
of  the  stomatal  pore,  as  was  observed  in  other  species,  and  outer  ridges 
as  well  as  vestibule  appear  to  be  quite  wanting.  Whether  such  are 
present  in  young  leaves,  however,  as  may  have  been  the  case,  was  not 
determined.  As  indicated  by  the  orange  color  following  the  use  of 
chloroiodide  of  zinc,  the  outer  portion  of  the  walls  of  the  guard-cells, 
to  a  point  slightly  within  the  stomatal  chamber,  is  cuticularized. 

The  chlorenchyma  consists  of  palisades,  rather  narrow  and  long. 
Within  this  tissue  are  elongated  but  wide  cells  which  are  radially 
disposed,  and  again  within  the  last  are  cuboid  cells.  The  last  are  not 
chlorophyll-bearing.  The  walls  are  fairly  heavy  and  there  are  numer¬ 
ous  intercellular  spaces.  Sclerenchymatous  fibers  are  present  on 
either  side  of  the  fibro-vascular  bundle,  that  on  the  ventral  side  being 
especially  well  developed. 

Euryops  lateriflorus. 

Euryops  lateriflorus,  one  of  the  species  used  in  the  studies  on  the 
transpiration  power  of  leaves,  was  collected  by  a  streamway  at  Mat- 
jesfontein.  The  habit  of  the  species  is  shown  in  plate  10b. 

Dried  material  only  was  used  in  the  anatomical  study.  The  cori¬ 
aceous  leaves  are  about  6  by  15  mm.  in  size.  They  are  fairly  abundant 
and  are  appressed  to  the  upright  branches. 

An  examination  of  the  leaf  in  cross-section  shows  the  following 
noteworthy  features :  The  epidermis  has  a  very  much  thickened  outer 
wall.  The  palisade  tissue,  which  is  similar  on  both  sides  of  the  leaf, 
extends  to  the  middle  of  the  leaf  and  abuts  on  one  or  two  layers  of 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


99 


thick-walled  cuboid  cells.  Rather  small  groups  of  sclerenchyma  occur 
in  connection  with  the  fibro-vascular  bundles,  particularly  the  larger 
ones,  and  on  the  ventral  side.  Surrounding  the  conductive  tissue, 
and  in  the  larger  bundles  between  it  and  the  dorsal  epidermis,  are  thick- 
walled  cuboid  cells.  Large  canals,  possibly  resiniferous,  are  scattered 
through  the  chlorenchyma,  mainly,  however,  appearing  on  the  ventral 
side  of  the  supporting  tissue,  or,  when  this  is  wanting,  in  association 
with  the  phloem.  The  extremely  heavy  outer  epidermal  wall,  to¬ 
gether  with  the  occasional  strands  of  sclerenchymatous  fibers,  account 
for  the  leathery  texture  of  the  leaves. 

Stomata  occur  on  both  surfaces  of  the  leaves.  The  guard-cells  are 
deeply  sunken  and  the  walls  lining  the  outer  stomatal  tube,  together 
with  the  outer  walls  of  the  guard-cells,  are  heavy  and  probably  cuticu- 
larized,  as  indicated  by  reaction  with  the  use  of  an  alcoholic  solution  of 
chlorophyll,  staining  deep  green.  The  opening  of  the  outer  stomatal 
tube  is  much  constricted,  and  the  outer  vestibule  ridges  are  well 
developed,  so  that  the  approach  to  the  stomata  is  guarded  by  small 
pores  of  fixed  diameter  (fig.  10b). 

Pentzia  virgata. 

Pentzia  virgata  was  collected  at  Matjesfontein,  where  it  occurs 
abundantly  on  the  veld.  It  was  found  to  be  dominant,  with  Mesem- 
bryanthemum  spinosum,  in  quadrat  No.  4,  the  vegetational  character 
of  which  is  shown  in  plate  21a. 

The  leaves  of  Pentzia  are  about  1.5  mm.  in  length  and  are  terete. 
They  occur  in  groups  of  four  or  more  on  dwarf  shoots,  4  mm.,  more  or 
less,  in  length,  and  are  abundant. 

The  main  structural  features  consist  of  an  epidermis  with  relatively 
heavy  outer  wall,  and  trichomes,  both  cover  trichomes  and  glandular 
ones,  and  a  mesophyll  of  which  the  outer  layers  are  palisades  and  the 
inner  are  cuboid  cells.  The  former  only  contain  chlorophyll.  Glandu¬ 
lar  ducts  are  sparingly  present  in  the  mesophyll. 

The  stomata  occur  on  each  surface  of  the  leaf  and  are  about  flush 
with  the  general  level  of  the  epidermis  (fig.  10c).  In  young  leaves 
both  secreting  and  non-secreting  trichomes  are  abundant,  but  in  the 
older  leaves  the  latter  drop  away. 

PtERONIA  FLEXICAULI9. 

The  material  of  Pteronia  flexicaulis  used  in  this  study  was  collected 
at  Matjesfontein  in  quadrat  No.  4,  where  it  occurs  with  Mesembry- 
anthemum  spinosum ,  Pentzia  virgata,  Pteronia  incana ,  Crassula  colum - 
naris ,  Asparagus  capensis,  and  others  (plate  21a).  The  leaves,  or 
leaf-like  branches,  are  filiform,  measuring  about  1  by  30  mm.  They 
are  somewhat  rigid. 

The  leading  features  of  the  anatomy  can  be  briefly  presented  (fig.  10d). 
The  epidermis  is  heavy  because  of  the  extremely  thick  outer  wall. 


100 


FEATURES  OF  THE  VEGETATION  OF  THE 


The  stomata  are  not  noticeably  sunken,  but  are  fairly  flush  with  the 
general  level  of  the  leaf-surface  (fig.  10e).  Trichomes  are  present. 
The  chlorenchyma  is  of  palisades  and  in  diameter  about  equals  one- 
eighth  of  the  cross-section  of  the  leaf.  With  the  chlorenchyma  is 
a  central  mass  of  cuboid  cells  in  which  are  numerous  fibro-vascular 
bundles,  associated  with  which  are  heavy  strands  of  sclerenchyma. 
Supporting  tissue  also  appears  to  be  present,  unaccompanied  by  fibro- 
vascular  bundles  wholly  within  the  chlorenchyma. 

Pteronia  incana. 

Pteronia  incana  was  collected  at  Matjesfontein  in  quadrat  No.  4, 
where  it  occurs  with  other  species  mentioned  in  the  previous  para¬ 
graph  and  the  general  character  of  which  is  shown  in  plate  31b. 
Dried  material  only  was  used  in  the  anatomical  examination. 

The  leaves  occur  in  groups  of  a  half  dozen  or  more  and  measure 
about  1.5  by  7  mm.  When  young  they  are  terete  in  cross-section, 
but  when  mature  they  are  somewhat  flattened. 

A  semi-diagrammatic  section  of  an  old  leaf,  such  as  represented 
by  figure  10f,  shows  several  main  structural  features,  including  heavy 
epidermis,  ducts  (d),  palisade  chlorenchyma  (p),  a  non-chlorophyllous 
mesophyll  of  relatively  few  but  large  cells,  and  four  or  more  strands 
of  conductive  tissue  ( fv ). 

When  viewed  somewhat  more  closely,  it  is  seen  that  the  conductive 
tissue  is  accompanied  by  strands  of  heavy-walled  fibers  which  are 
especially  prominent  in  association  with  the  midrib  of  the  leaf,  and, 
among  other  features,  that  the  palisades  are  not  long  cells  and  the 
chlorenchyma  may  be  one  to  three  cell-layers  wide.  Stomata  are 
numerous  on  both  surfaces  and  are  not  deeply  placed  (fig.  10g).  The 
outer  wall  of  the  epidermis  is  very  heavy.  The  outermost  part  of  the 
wall  is  cuticularized,  but  for  the  most  part  it  appears  to  be  cellulose,  as 
would  be  indicated  by  the  reaction  to  chloroiodide  of  zinc.  Trichomes 
are  abundantly  present  in  young  leaves,  but  are  largely  wanting  in 
those  that  are  mature.  There  are  both  cover  and  glandular  trichomes, 
of  which  the  latter  appear  to  persist  the  longer  of  the  two. 

Relhania  squarrosa. 

Relhania  squarrosa  is  a  shrub  common  on  the  veld  in  the  vicinity 
of  Matjesfontein.  The  branches  are  strict  and  well  covered  with  small, 
saddle-shaped  and  somewhat  recurved  leaves.  They  stand  out  on  the 
older  parts  of  branches  nearly  at  right  angles  to  the  branch,  but  on 
the  younger  parts  they  may  be  appressed  to  the  branch.  There 
thus  is  a  distinctly  ventral  or  lower  side  as  opposed  to  the  distinctly 
upper  or  dorsal  side,  with  respect  to  the  position  of  the  branch,  and 
hence  with  regard  to  such  environmental  features  as  the  direction 
of  the  impinging  rays  of  light.  Such  features  would  appear  to  be 
associated  with  a  dorsi-ventral  structure. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


101 


The  leaves  are  about  4  by  8  mm.  in  size.  An  examination  of  a 
cross-section  shows  the  leaf  to  have  the  following  leading  structural 
features:  The  epidermis  has  a  very  heavy  outer  wall  of  about  the 
same  thickness  on  either  side  of  the  leaf.  Glandular  trichomes  occur 
on  both  dorsal  and  ventral  sides,  but  sparingly.  Where  they  take 
origin  the  outer  epidermal  wall,  as  is  usually  the  case,  is  not  thickened, 
so  that  the  trichomes  have  the  effect  of  being  somewhat  sunken  in  the 
leaf.  Stomata  occur  both  on  the  dorsal  and  the  ventral  side  and 
are  nearly  or  quite  flush  with  the  general  level  of  the  surface  of  the 
leaf.  The  chlorenchyma  of  both  dorsal  and  ventral  sides  consists 
of  palisade  cells  of  a  similar  appearance.  Sclerenchyma,  which  is  often 
to  be  found  in  connection  with  the  conductive  tissue  in  the  leaves, 
appears  to  be  quite  wanting,  so  that  the  leathery  character  is  largely 
owing  to  the  heavy  outer  epidermal  wall  in  addition  to  the  fibro- 
vascular  bundles. 

It  would  appear  from  the  foregoing  that  the  bifacial  appearance  is 
not  carried  out  in  the  structure  of  the  leaves. 

Stgebe  sp. 

The  material  of  Stoebe  sp.  used  in  the  anatomical  study  was  col¬ 
lected  about  0.5  mile  west  of  Matjesfontein,  where  it  occurs  in  associa¬ 
tion  with  several  other  perennial  shrubs,  including,  among  others, 
Aster  filifolius,  Cotyledon  sp.,  Eriocephalus  sp.,  and  Pelargonium  sp. 

The  leaves  are  numerous  but  small,  measuring  about  1  by  10  mm., 
and  occur  in  groups  of  a  half  dozen  or  more.  The  young  leaves  are 
somewhat  folded,  so  that  the  surfaces  on  either  side  of  the  midrib, 
dorsal  side,  approximate  one  another. 

In  cross-section  the  leaf  shows  marked  structural  characteristics. 
On  the  ventral  side  cover  trichomes  occur  in  great  abundance,  forming 
a  felt  whose  situation  and  extent  are  indicated  diagrammatically  in 
figure  lOi.  They  are  apparently  never  so  abundant  on  the  dorsal 
side,  and  in  the  mature  leaves  they  are  mainly  wanting  on  that  side, 
although  still  remaining  on  the  ventral  side.  The  epidermis  on  the 
dorsal  side  has  a  very  heavy  outer  wall,  while  that  of  the  opposite 
side  is  light.  In  the  former  instance,  through  secondary  thickening 
of  the  wall,  as  was  found  to  be  the  case  in  Eriocephalus  and  shown 
in  figure  10j,  the  lumen  of  the  cells  of  that  side  has  nearly  or  wholly 
disappeared.  Stomata  are  to  be  found  only  on  the  ventral  side  and 
present  the  peculiarity  of  having  the  guard-cells  sharply  projecting 
above  the  level  of  the  leaf.  The  chlorenchyma  is  of  palisade  cells 
throughout,  although  that  of  the  ventral  side  is  possibly  to  be  more 
accurately  described  as  being  palisade-like,  inasmuch  as  the  cells 
are  not  so  long,  or  at  least  relatively  more  stout  than  the  corresponding 
cells  of  the  other  side.  Between  the  dorsal  and  the  ventral  chloro¬ 
phyll-bearing  cells,  and  hence  in  the  middle  of  the  leaf,  are  about  two 


102 


FEATURES  OF  THE  VEGETATION  OF  THE 


layers  of  cuboid  cells  which  do  not  contain  chlorophyll.  Scleren- 
chyma  is  strongly  developed  in  relation  to  the  conductive  tissue,  and 
is  especially  heavy  on  the  ventral  side  of  the  bundles. 

GENERAL  SUMMARY  AND  DISCUSSION  OF  LEAF-STRUCTURES. 

Although  the  list  of  species  of  which  the  foliar  structure  has  just 
been  passed  in  review  is  not  a  long  one,  the  number  of  families  repre¬ 
sented  is  considerable.  They  are  as  follows:  Liliacese,  Proteacese, 
Menispermacese,  Alizoceae,  Capparidacese,  TiliaceaB,  Anacardiaceae, 
Celastraceae,  Araliaceae,  Leguminosae,  Crassulaceae,  Labiatae,  Scro- 
phulariaceae,  Asclepiadaceae,  Apocynaceae,  Ebenaceae,  and  Compositae. 
A  glance  at  the  list  of  species  observed  during  the  course  of  the  recon¬ 
naissance  of  the  plants  of  the  more  arid  portions  of  southern  Africa, 
given  on  page  10,  will  show  that  the  list  of  families  represented  by  the 
species  observed  might  have  been  considerably  extended.  Although 
not  to  have  done  so  is  to  be  regretted,  it  was  not  feasible  from  the  fact, 
as  suggested  before,  that  many  of  the  forms  did  not  have  sufficient 
supporting  tissue  to  satisfactorily  maintain  the  structures  for  studies 
of  an  anatomical  nature,  which  were  largely  carried  on  with  dried 
material.  When  there  is  a  relatively  large  amount  of  tissue  with  heavy 
walls  present,  especially  fibrous  tissue,  the  implication  is  that  species 
having  such  tissues  may  have  developed  under  arid  conditions,  inas¬ 
much  as  under  such  conditions  there  may  be  organized  a  greater 
amount  of  cellulose  and  a  smaller  amount  of  starch  than  when  the 
environment  is  humid.  Moreover,  in  such  an  intensely  arid  region 
as  central  Australia,  for  example,  the  great  abundance  of  sclerenchyma 
in  foliar  organs  constitutes  an  important  item  in  the  support  of  such  a 
view.1  It  then  is  an  interesting  circumstance  that  many  species  of  the 
more  arid  portions  of  southern  Africa  are  poor  in  the  structural  feature 
referred  to.  Without  going  further  into  this  phase  of  the  general 
question  in  this  place,  it  can  be  pointed  out  that  although  many  of  the 
species  have  developed  along  the  alternative  line,  that  is,  they  are  able 
to  organize  mucilages,  there  appear  to  remain  species,  not  annuals, 
in  which  the  power  of  converting  polysaccharides  into  anhydrides  is 
not  especially  well  marked.  It  is  such  forms,  along  with  those  that 
are  succulent,  with  one  or  two  exceptions,  that  have  been  omitted 
per  force  in  the  present  study  of  foliar  structures. 

A  glance  at  the  sketch  of  the  more  important  or  more  striking 
features  of  the  foliar  structures,  as  given  in  the  preceding  section, 
will  show  relatively  little  uniformity  in  structural  relations,  although 
some  important  relations  will  appear.  It  will  be  seen  that  by  no 
means  all  of  the  features  are  to  be  traced  to  the  probable  immediate 
influence  of  an  arid  environment,  or  at  least  the  relation  must  be  said 

1  Plant  habits  and  habitats  in  the  arid  portions  of  South  Australia.  W.  A.  Cannon.  Pub¬ 
lication  No.  308,  Carnegie  Inst.  Wash.,  1921,  pp.  32  and  136. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


103 


to  be  obscure.  At  the  same  time  it  is  borne  in  mind  that  here  a  single 
plant  organ  is  dealt  with,  and  not  the  root  and  shoot  with  the  con¬ 
stituent  organs;  consequently  judgment  must  be  made  with  caution. 
However,  certain  deductions  can  be  made  which  may  be  of  interest, 
even  with  the  limitations  just  implied. 

In  comparative  studies  on  plant  anatomy,  from  a  physiological 
point  of  view,  when  an  attempt  is  made  to  account  if  possible  for 
peculiarities  in  structure,  it  is  recognized  that  two  different  and 
important  groups  of  forces  are  to  be  considered.  These  relate  to  the 
influence  of  heredity  and  to  that  of  the  immediate  environment.  In 
studies  on  desert  plants  the  latter  has  often  been  stressed,  doubtless 
owing  to  the  spectacular  association  of  perennials  with  conditions 
of  little  rain,  when,  especially  to  the  physiologist,  possibly  little 
accustomed  to  such  an  environment,  survival  appears  to  be  little  less 
than  miraculous.  In  such  forms,  as  well  as  in  those  having  more 
favorable  moisture  conditions,  it  is  axiomatic  that  some  consideration 
should  be  paid  to  influences  outside  of  and  apart  from  that  of  the  imme¬ 
diate  environment  as  well  as  to  the  latter.  It  is  clearly  impracticable 
to  make  direct  comparison  with  ancestral  forms,  but  it  is  not  impossible 
to  compare  the  structure  of  whatever  species  desirable  with  that  of 
other  members  of  the  family  which  may  be  living  under  less  arid  con¬ 
ditions  for  the  purpose  of  drawing  conclusions  as  to  morphological 
adjustments  which  may  have  been  occasioned  by  such  environment. 
Even  so,  and  as  will  occur  to  every  one,  there  will  be  lacunae  difficult 
or  impossible  to  bridge,  which  may  make  conclusions  tentative  and 
subject  to  revision.  However,  possible  suggestions,  taken  as  such, 
may  be  worth  while  as  a  means  of  throwing  some  light  on  the  direc¬ 
tions  of  morphological  adjustments,  if  in  no  other  way. 

Epidermis. 

The  epidermal  cells  of  the  plants  examined  vary  greatly  in  form. 
Thus  some  are  squat  and  others  are  much  elongated  in  a  direction  at 
right  angles  to  the  leaf-surface.  The  most  striking  of  the  epidermal 
cells  observed  were  the  vesicular  ones  of  Galenia  africana ,  which 
constitute  a  trichomelike  covering  of  the  leaf.  The  epidermal  cells 
vary  greatly  also  in  size.  This  can  be  seen  by  an  examination  of 
the  figures  for  Aloe,  Asparagus ,  Cadaba,  Cussonia,  Eriocephalus,  Galenia , 
Protea ,  and  Stachys,  all  of  which  are  drawn  to  the  same  magnification. 
In  the  case  of  Grewia  cana,  the  epidermal  cells  of  the  two  sides  of  one 
and  the  same  leaf  are  of  unequal  size.  Thus  the  area  of  a  dorsal 
epidermal  cell,  in  section,  may  be  7.6  times  that  of  a  ventral  epi¬ 
dermal  cell  directly  opposite,  and  the  extreme  difference  in  size  of 
cells  of  the  two  surfaces  is  much  greater  than  this.  Thus,  there  is 
little  uniformity  among  these  xerophytes  as  regards  the  cell  characters 
above  referred  to. 


104 


FEATURES  OF  THE  VEGETATION  OF  THE 


Possibly  the  most  striking  single  anatomical  character  of  foliar 
organs  common  to  perennials  of  an  arid  region  is  the  possession  of 
a  heavy  outer  epidermal  wall.  This  feature  seems  to  hold  in  all 
instances,  except  only  in  such  species  as  have  a  permanent  cover  of 
trichomes.  In  such  case,  as  in  Stachys  sp.,  the  outer  wall  of  the 
epidermis  is  thin.  Where,  also,  a  portion  of  the  leaf  only  bears  tri¬ 
chomes,  as  in  Grewia  and  Stoebe,  the  exposed  portion  has  a  heavy  outer 
wall,  but  in  the  protected  part  the  outer  wall  is  thin.  That  the  ex¬ 
posed  outer  wall  of  xerophytic  perennials  should  be  heavy  is  not 
difficult  to  understand  when  it  is  recalled  that  such  portions  of  the 
plant  most  intimately  connected  with  the  arid  environment  are  most 
subject  to  drying,  and  that  under  such  conditions  anhydrides  are 
formed,  as  mentioned  at  the  beginning  of  the  present  section. 

The  general  occurrence  of  a  heavy  outer  epidermal  wall  in  xero- 
phytes,  with  the  exceptions  noted  where  the  epidermal  cells  are  not 
immediately  in  contact  with  the  drying  atmosphere,  is  therefore  a 
characteristic  of  plants  of  this  class.  So  far  as  the  inner  epidermal 
wall  and  the  lateral  walls  are  concerned,  however,  no  such  uniformity 
exists.  The  lateral  walls  appear  to  be  variable  in  at  least  two  direc¬ 
tions,  namely,  as  regards  thickness  and  as  regards  the  quality  of  being 
curved,  undulating,  or  plane.  Solereder  states  that  the  margins  of 
epidermal  cells  of  species  of  dry  habitats  are  straight,  while  analogous 
cell-walls  of  species  from  moist  habitats  are  wavy.1  But  the  case  is 
apparently  not  so  simple  as  this.  In  mesophytic  dicotyls  the  lateral 
walls  are  commonly  straight  on  the  dorsal  leaf-surface  and  wavy  on 
the  ventral  surface,  and  waviness  culminates  in  mesophytic  species. 
But  in  xerophytic  and  hydrophytic  dicotyls  and  generally  in  mono- 
cotyls  the  side- walls  are  usually  straight.2  But  the  species  used  in 
connection  with  the  present  paper  were  not  studied  in  this  particular. 
So  far  as  the  thickness  of  the  inner  and  the  lateral  epidermal  walls  are 
concerned,  especially  the  latter,  there  is  probably  much  variation, 
possibly  within  the  species,  certainly  as  between  different  species,  and 
there  is  possibly  no  clear  concordance  between  thickness  and  the 
character  of  the  environment.  The  extreme  in  the  formation  of 
thick  lateral  walls  appears  in  Eriocephalus  sp.,  in  which,  mainly 
because  of  the  extreme  thickness  of  these  walls,  the  cell  lumen  is 
nearly  or  entirely  obliterated.  A  similar  condition  appears  to  obtain 
in  the  ventral  epidermal  cells  of  Stoebe.  As  regards  the  increase  in 
thickness  of  the  lateral  walls,  it  is  perhaps  easier  to  explain  why  they 
should  be  greatly  thickened  in  an  arid  habitat  than  that  they  should 
be  subject  to  drying  conditions  and  still  not  always  suffer  the  fate 
of  the  outer  wall. 

1  L.  c.  p.  905. 

2  A  textbook  of  botany.  Vol.  2,  Ecology.  Coulter,  Barnes,  and  Cowles,  p.  571.  See,  also, 
Haberlandt,  Physiologische  Pflanzenanatomie,  Leipzig,  1904,  p.  104. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


105 


Another  noteworthy  feature  of  the  species  studied  is  the  general 
cuticularization  of  all  or  a  portion  of  the  outer  wall.  The  deposition 
of  cutin  is  closely  associated  with  the  transpiratory  activity  of  the  leaf 
and  is  not  limited  to  xerophytes.  Cutinization  is  less  marked  on  the 
under  surface  of  the  leaf  and  in  stomatal  pits,  and  also  is  less  in  plants 
growing  in  protected  situations  than  in  situations  that  are  exposed.1 
So  far  as  was  observed,  the  deposition  of  cutin  is  mainly,  usually  ex¬ 
clusively,  in  the  outer  epidermal  wall,  and  not  in  the  side-walls  or  the 
inner  wall.  As  an  exception,  however,  to  the  last  remark  it  was  seen 
that  the  lateral  epidermal  walls  in  Cussonia  spicata  give  the  same 
reaction  with  chloroiodide  of  zinc  as  the  outer  wall,  and  are  probably 
cuticularized.  As  regards  the  stomatal  pores,  the  lining  walls  up  to  the 
guard-cells,  and  including  the  outer  walls  of  the  latter,  may  be  cutic¬ 
ularized.  The  heaviest  deposition  of  cutin  was  seen  in  the  outer 
epidermal  wall  in  Asparagus  striatus  and  in  Cussonia  spicata ,  in  which 
the  unmodified  cellulose  constitutes  only  a  thin  inner  lining  of  the  wall. 

Yet  another  feature  of  the  epidermis  which  should  be  mentioned 
is  the  occasional  presence  of  a  waxy  cover  on  the  cuticle.  It  was 
observed  in  Galenia  africana,  Royena  pollens ,  and  Asclepias  filiformis 
(?)  as  a  granular  coat.  As  in  the  case  of  the  formation  of  cutin,  that 
of  wax  is  not  confined  to  xerophytes,  although  it  is  best  developed  in 
plants  of  this  class.  Unlike  cutin,  wax  is  heaviest  on  the  under  sur¬ 
face  of  leaves,  but,  like  cutin,  it  is  best  developed  under  conditions  of 
excessive  transpiration.  The  point  is  made  that  the  formation  of  wax, 
or  resin,  as  an  outer  coating  of  the  foliar  organs  of  the  species  studied 
can  not  be  said  to  be  the  usual  occurrence,  but,  on  the  other  hand,  it 
appears  to  be  fairly  rare.  This  is  rather  surprising  from  the  known 
fact  that  a  waxy  covering  of  the  leaves  cuts  down  the  loss  of  water  very 
considerably.  Thus,  Haberlandt2  cites  the  results  of  experiments 
which  indicate  that  leaves  with  the  natural  coating  of  wax  carefully 
removed  evaporate  from  1.1  to  1.5  times  more  water  in  unit  time 
and  for  unit  leaf  area  than  the  same  leaves  with  the  wax  in  place. 

Although  not  observed  in  the  species  here  considered,  it  is  known 
that  the  cuticularized  portion  of  the  outer  epidermal  wTall,  in  the  leaves 
of  Mesembryanthemum  sp.3 4  and  of  Wehcitschia  mirabilisf  may  contain 
minute  crystals  of  calcium  oxalate.  Larger  crystals  of  different  form 
are  present,  often  in  great  amount,  in  different  parts  of  the  leaves  in 
most  of  the  species  examined.  The  inclusion  of  inorganic  substances, 
including  calcium  oxalate,  in  old  cell-membranes  is  apparently  not 
restricted  to  the  epidermis  or  to  species  which  inhabit  arid  habitats. 

In  no  species  was  the  inner  epidermal  wall  found  to  be  at  all  com- 

1  A  Textbook  of  Botany.  Vol.  2,  Ecology.  Coulter,  Barnes,  and  Cowles,  p.  569. 

2  Physiologische  Pflanzenanatomie,  G.  Haberlandt,  Leipzig,  1904,  p.  99. 

3  Solereder,  l.  c.,  p.  470. 

4  The  anatomy  and  morphology  of  the  leaves  and  infloresences  of  Welwitschia  mirabilis,  M. 

G.  Sykes.  Phil.  Trans.  Roy.  Soc.  London,  Ser.  B.,  vol.  201,  p.  181.  1910. 


106 


FEATURES  OF  THE  VEGETATION  OF  THE 


parable  to  the  outer  wall  in  thickness,  and  usually  it  is  relatively  or 
actually  thin.  Only  in  the  case  of  Cussonia  spicata  can  the  inner  wall 
of  the  outer  cell-layer  be  said  to  be  heavy,  and  in  this  instance  the 
comparison  falls  down,  for  the  reason  that  the  epidermis  of  Cussonia 
is  several  cells  in  thickness.  However,  the  physiological  inner  epi¬ 
dermal  wall,  which  lies  several  cells  beneath  the  outer  epidermal  layer, 
is,  in  this  species,  fairly  heavy,  although  pores  do  not  appear  to  be 
present.  The  inner  wall  is  apparently  never  impregnated  with  cutin, 
which  would  hinder  the  free  passage  of  water,  and  material  in  solution, 
to  or  from  the  epidermal  cell.  The  inner  wall  of  all  species  examined 
in  this  particular  gave  the  cellulose  reaction  with  suitable  reagents,  and 
no  indications  were  seen  of  its  being  converted  into  mucilages,  although 
mucilaginous  inner  epidermal  walls  are  known  in  species  of  the  Lilia- 
cese,  Tiliacese,  and  Leguminosse. 

In  Cussonia  spicata  and  in  Gymnosporia  buxifolia  the  epidermis, 
dorsal  in  the  species  first  named,  consists  of  more  than  one  cell-layer.1 
In  Cussonia  the  hypoderm  is  several  cells  thickness  and  constitutes 
an  important  portion  of  the  mechanical  tissues  of  the  leaf,  but  in 
Gymnosporia  it  is  only  one  cell  in  depth.  In  the  latter  species  the 
hypoderm  may  contain  crystals  of  calcium  oxalate,  although  many 
of  the  cells  have  heavy  walls  and  are  provided  with  pits. 

Solereder  (Z.  c.,  p.  910)  gives  a  list  of  families  of  which  some  species 
may  have  a  hypoderm.  Among  these,  and  in  addition  to  the  Tiliacese, 
to  which  Gymnosporia  belongs,  and  the  Araliacese,  of  which  Cussonia  is 
a  member,  and  which  are  represented  by  species  whose  structure  is 
mentioned  in  the  present  paper,  are  the  following:  Apocynacese, 
Asclepiadacese,  Capparidaceae,  Compositse,  Leguminosse,  Protacese, 
and  Scrophulariacese.  In  the  two  species  only,  however,  as  mentioned 
above,  was  hypoderm  found. 


Stomata. 

The  distribution  of  the  stomata  on  the  leaf  and  the  position  of  the 
guard-cells  with  relation  to  the  general  level  of  the  leaf-surface  are 
various  and  have  different  apparent  correlations.  But  among  other 
variable  features,  more  or  less  related  to  that  last  named,  may  be 
mentioned  the  development  of  the  vestibule,  as  well  as  that,  in  some 
instances,  of  an  outer  vestibule,  the  stomatal  pit,  also.  It  will  be  seen 
that  the  presence  of  certain  of  these  features  is  in  apparent  relation  to 
the  fact  of  an  arid  environment,  although  the  relation  may  not  be 
immediate  and  direct. 

The  distribution  of  the  stomata  as  between  the  dorsal  and  ventral 
surfaces  follows  in  general  the  symmetry  of  the  leaf-structure.  That 
is,  in  dorsi-ventral  leaves  the  stomata  are  confined  to  the  ventral 

1  In  this  account  no  distinction  is  made  between  hypoderm  and  multicellular  epidermis, 
although  the  two  have  unlike  origins.  The  physiological  role  played  by  them  may  be  similar. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


107 


surface,  otherwise  they  are  on  both  surfaces.  In  the  case  of  Stoebe  sp., 
however,  where  the  chlorenchyma  of  the  ventral  side  is  palisade-like, 
and  hence  in  which  the  symmetry  is  modified  dorsi-ventral,  the 
stomata  are  nevertheless  confined  to  the  ventral  surface. 

The  relative  number  of  stomata  was  observed  in  one  case,  namely, 
Cotyledon  paniculata,  where  comparison  was  made  with  the  number  in 
a  cultivated  variety  of  the  sunflower.  In  this  instance  there  was  ap¬ 
proximately  twice  as  many  stomata  per  unit  area  in  the  sunflower  leaf 
as  in  that  of  the  Cotyledon. 

The  guard-cells  of  stomata  of  the  species  examined  lie  in  some 
below  the  general  surface  of  the  leaf,  in  other  species  somewhat  above, 
and  in  other  species  they  are  fairly  flush  with  the  leaf-surface.  In 
such  stomata  as  are  more  or  less  deeply  placed,  the  depth  of  the  guard- 
cells  is  in  large  part  dependent  on  the  thickness  of  the  outer  epidermal 
wall,  although  there  are  exceptions  to  this  statement.  Thus  in  Cadaba 
juncea  it  is  the  depth  of  the  epidermis  as  a  whole  that  determines  that 
of  the  stomata,  and  a  similar  condition  obtains  in  Euyrops  lateri- 
florus,  as  well  as  in  Protea  neriifolia.  It  was  found,  however,  in  many 
species  that  the  guard-cells  were  not  deeply  placed,  although  the  outer 
epidermal  wall  might  be  fairly  heavy.  As  examples  of  the  last,  Anti- 
zoma  capensis,  Aptosimum  indivisum,  and  Carissa  ferox  can  be  men¬ 
tioned.  In  Stachys  sp.  and  Stoebe  sp.  the  guard-cells  project  some¬ 
what  above  the  general  level  of  the  leaf.  In  the  species  last  named 
there  is  a  heavy  and  permanent  cover  of  trichomes  on  the  portion  of 
the  leaf  where  the  stomata  are  situated. 

The  outer  guard-cell  ridges,  which  delimit  the  outer  portion  of  the 
outer  vestibule,  are  well  developed  in  some  species,  as  in  Aptosimum 
indivisum,  Antizoma  capensis,  and  Cadaba  juncea,  and  may  be  in  others 
as  well.  They  serve  as  a  means  of  constricting  the  approach  to  the 
stoma  and  in  many  species  constitute  the  only  protection,  of  this  sort, 
to  it.  In  such,  however,  as  have  deeply  placed  stomata  the  very 
entrance  to  the  stomatal  pit,  or  chamber,  may  be  constricted  in  such 
way  and  to  such  an  extent  as  to  make  possible  the  delimiting  of  a 
second  outer  vestibule,  as  in  Euryops  lateriflorus  and  Protea  neriifolia. 
The  guard-cell  ridges  and  the  inner  portion  of  the  secondary  vesti¬ 
bule  are  cuticularized. 

Trichomes. 

Trichomes  were  found  in  12  of  the  27  species  whose  leaves  were 
studied,  and  may  possibly  be  found  in  young  leaves  of  others.  In 
Stachys,  trichomes  'cover  the  mature  leaf  and  in  Stoebe  they  are  present 
on  the  ventral  surface  only  and  are  persistent.  But  for  the  most  of  the 
species  with  trichomes  the  cover  trichomes,  at  least,  fall  away  in  age. 
It  can  be  rightly  concluded,  therefore,  that,  so  far  as  the  species  re¬ 
ferred  to  are  concerned,  the  occurrence  of  trichomes  is  not  a  marked 


108 


FEATURES  OF  THE  VEGETATION  OF  THE 


characteristic  or  one  that  is  particularly  to  be  associated  with  an  arid 
habitat.  It  should  be  remarked,  also,  that  in  few,  if  any,  of  the  species 
studied  was  there  found  excessive  development  of  glandular  trichomes, 
or  evidences  of  very  active  secretion  on  their  part  (but  see  Rhus  sp. 
above).  There  was  nothing,  for  example,  at  all  comparable  to  the 
condition  which  was  found  in  the  genus  Eremophila  in  southern 
Australia,  and  in  E.  freelingii  in  particular,  where  the  exudation  from 
the  trichomes  quite  covered  the  glandular  hairs  themselves.1 

In  Eriocephalus  sp.,  the  base  of  the  cover  trichomes  is  cuticularized, 
while  the  middle  and  the  distal  portions  are  unmodified  cellulose,  and 
this  seems  to  be  the  condition  of  the  trichomes  in  several  other  species 
as  well. 

The  correlation  of  a  covering  of  trichomes  and  a  thin  outer  epidermal 
wall  has  been  commented  on  in  a  preceding  paragraph.  (See  also, 
Euclea  undulata  in  this  regard.) 

Mesophyll. 

The  fundamental  tissue  of  the  leaves  of  the  species  examined  varies 
in  the  direction  and  in  the  degree  of  development  and  apparently  is 
somewhat  less  directly  affected  by  the  arid  conditions  of  the  environ¬ 
ment  than  are  the  integumentary  tissues.  However  this  may  be, 
there  are  trends  in  development  which  appear  to  have  correlations 
with  the  more  striking  environmental  factors  which  indicate  possible 
degrees  of  “ usefulness”  if  not  of  direct  cause  and  effect;  and  certain  of 
these  can  be  mentioned  in  this  summary  of  the  main  structural  char¬ 
acteristics  of  the  mesophyll. 

Of  the  species  examined,  18  were  observed  to  have  the  same  kind  of 
chlorophyll-bearing  cells  on  the  two  sides  of  the  leaf,  that  is,  they  are 
isosymmetrical,  6  have  well-defined  dorsi-ventral  structural  sym¬ 
metry,  and  in  3  the  symmetry  is  intermediate.  Where  the  structure 
is  symmetrical,  the  chlorenchyma  is  of  palisades,  except  in  the  case 
of  succulents,  as  Aloe  variegata,  for  example,  where  the  outer  layer 
of  the  mesophyll  is  neither  cuboid  nor  palisade,  but  is  somewhat 
elongated  in  a  direction  away  from  the  surface  of  the  leaf.  It  appears, 
therefore,  that  for  the  most  part  the  chlorenchyma  of  The  species 
studied  is  composed  of  cells  whose  longest  axis  is  at  right  angles  to 
the  leaf-surface. 

Although  the  material  available  for  study  was  for  the  most  part  not 
in  condition  suitable  for  determining  the  point  as  satisfactorily  as 
would  be  desired,  it  was  observed,  however,  that  the  extent  of  the 
intercellular  spaces  of  the  chlorenchyma  was  exceedingly  varied; 
and  further,  that  in  palisade  chlorenchyma  the  intercellular  spaces  are 
relatively  small,  being  smaller  in  well-developed  than  in  poorly- 
developed  or  modified  palisade  tissue;  and  they  are  best  developed  in 


1  Plant  habits  and  habitats  in  the  more  arid  portions  of  South  Australia,  q.  v.,  p.  128. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


109 


spongy  parenchyma,  that  is,  in  species  having  dorsi-ventral  structural 
symmetry  of  the  leaves.  As  to  a  possible  correlation  of  this  condition 
with  the  aridity  of  the  habitat,  and  there  is  clearly  correlation,  there 
are  striking  exceptions.  For  example,  Asclepias  filiformis  (?)  occurs 
in  the  habitat  of  Welwitschia  mirabilis,  in  the  Namib  Desert.  The 
structure  of  the  leaf  is  dorsi-ventral.  But  Protea  neriifolia  is  to  be 
found  when  the  rain  is  very  considerable,  even  if  there  are  fairly  long 
periods  without  rain,  and  still  the  two  sides  of  the  leaf  are  apparently 
alike  in  structure.  Further  than  this,  species  having  cuboid  chloren- 
chyma,  and  those  with  chlorenchyma  which  is  of  palisades,  may  grow 
in  close  proximity.  But,  it  is  to  be  remarked,  the  first  of  these  may  be 
succulent  and  the  last,  apparently  with  no  exception,  is  sclerophyllous. 
The  well-marked  difference  in  metabolic  processes,  as  between  the  two 
classes  of  plants,  very  evidently  overcomes  possible  direct  morphogenic 
effect  of  light  on  the  chlorophyll-bearing  cells. 

Reactions  with  the  use  of  chloroiodide  of  zinc  indicate  that  the 
cuboid  chlorenchyma  of  Aloe  variegata  contains  mucilages.  In  many 
sclerophyllous  species  calcium  oxalate  in  crystals  is  to  be  found  in  the 
chlorenchyma,  or  other  cells,  of  the  mesophyll;  and,  finally,  in  the 
idioblasts  of  Cadaba  juncea,  which  occur  to  a  certain  extent  in  the 
chlorenchyma,  there  may  be  mucrosin  (?). 

Heavy-walled  tissues,  either  fibrous  or  isodiametric,  are  often 
marked  characteristics  of  xerophytes.  Such  tissues  were  found  to  be 
prominently  developed  in  many  perennials  of  South  Australia.1  Under 
such  conditions  the  amount  of  living  tissue  is  very  markedly  reduced, 
and  with  this  as  a  result  the  water  requirement,  in  proportion  to  the 
mass  of  the  plant,  becomes  small. 

The  mechanical  tissue  is  commonly  importantly  developed  in  rela¬ 
tion  to  the  fibro-vascular  bundles,  where  it  occurs  dorsally  and  ven- 
trally,  sometimes  (as  in  Euclea  undulata)  reaching  nearly  to  the  epi¬ 
dermis.  It  is  especially  marked  in  association  with  the  midrib,  but 
a  few  strands  of  fibers  are  often  to  be  found  with  the  smaller  strands 
of  conductive  tissue.  In  Asparagus  striatus  leaves  (?)  the  supporting 
tissue  constitutes  a  ring  which  separates  the  chlorenchyma  from  the 
more  deeply  lying  fundamental  and  conductive  tissues.  Of  the  species 
studied  in  this  regard,  6  were  not  noted  to  have  sclerenchymatous 
elements  of  the  types  referred  to,  but  in  16  species  supporting  tissue, 
fibrous,  was  present. 

Conductive  Tissues. 

Although  the  conductive  tissues  were  not  especially  studied,  it  was 
evident,  from  casual  inspection,  that  those  of  the  foliar  organs  vary 
in  a  broad  wa}^  and  in  a  manner  characteristic  of  the  type  of  plant  or 

1  Plants  and  plant  habitats  in  the  more  arid  portions  of  South  Australia.  W.  A.  Cannon, 
q.  v.,  p.  136. 


110 


FEATURES  OF  THE  VEGETATION  OF  THE 


of  leaf.  Thus  it  was  noted  that  the  mass  of  tissue  of  this  character, 
as  related  to  the  mass  of  the  chlorenchyma,  was  very  meager  in  Aloe 
variegata  and  Cotyledon  paniculata,  but  that  it  was  relatively  abundant 
in  all  sclerophylls. 

NOTES  ON  THE  ORIGIN  OF  FOLIAR  STRUCTURES. 

A  cursory  glance  over  the  most  striking  structural  features  of  the 
leaves  of  the  xeroph}Tes  mentioned  in  this  paper  leads  to  the  inevitable 
conclusion  that,  at  least  as  regards  the  species  studied,  very  diverse 
morphological  roads  have  been  followed  during  the  long  processses 
of  adjustment  to  their  respective  environments,  a  leading  character¬ 
istic  of  each  of  which  is  the  greater  or  smaller  degree  of  aridity.  But 
it  is  also  recognized,  as  mentioned  in  an  earlier  paragraph,  that  the 
foliar  organs  are  only  a  portion  of  the  “plant,”  and  that  to  have  a 
satisfactory  conception  of  the  morphological  relation  of  the  plant  to 
the  environment  it  is  also  necessary  to  take  into  consideration  the 
nature  of  the  development  of  the  root  as  well  as  of  the  shoot  in  its 
entirety,  including  the  leaves;  but  in  the  present  study  it  is  not 
possible  to  do  this.  The  writer,  therefore,  has  been  obliged  to  be 
content  with  comparing  the  structure  of  the  leaves  of  different  species 
belonging  for  the  most  part  to  unlike  families.  Conclusions  based  on 
such  meager  foundation  are  necessarily  tentative,  but,  nevertheless,  it 
is  believed  that  they  are  sufficiently  interesting  and  suggestive  to  be 
worth  undertaking.  In  order  to  estimate  the  possible  influence  on 
leaf-structure  of  family  characteristics,  and  indirectly  of  inherited 
qualities,  a  running  summary  of  the  leading  morphological  features  of 
the  leaves  of  the  families  here  represented  will  be  given  in  the  following 
few  pages,  together  with  something  of  their  geographical  distribution, 
especially  as  regards  southern  Africa,  and  a  short  recapitulation  of 
specific  structures  as  given  somewhat  in  detail  above.1  The  imme¬ 
diate  result  of  such  a  comparison  should  place  the  direct  effect  of  the 
environment  on  the  morphology,  especially  the  inner  morphology, 
of  the  leaf  in  all  the  sharper  relief. 

PROTACEiE. 

The  Protacese,  although  occurring  rarely  in  South  America  and 
New  Zealand,  are  especially  abundant  in  southern  Africa  and  Australia. 

1  Authorities  generally  available  were  used  in  determining  the  general  distribution  of  the 
families,  and  these  were  supplemented  by  recent  publications  by  South  African  writers,  among 
which  are  the  following:  The  flora  of  Natal  and  Zululand,  J.  W.  Bews,  Pietermaritzburg,  1921; 
Some  general  principles  of  plant  distribution  as  illustrated  by  the  South  African  flora,  J.  W. 
Bews,  Ann.  Bot.,  vol.  35,  1921;  Plant  succession  and  plant  distribution  in  South  Africa,  J.  W. 
Bews,  Ann.  Bot.,  vol.  34,  1920;  Phanerogamic  flora  of  the  divisions  of  Uitenhage  and  Port  Eliza¬ 
beth,  S.  Schonland,  Bot.  Sur.  So.  Africa,  Mem.  No.  1,  1919;  Das  Kapland,  R.  Marloth,  1908; 
The  veld:  Its  resources  and  dangers,  I.  B.  Pole  Evans,  So.  Af.  Jour.  Science,  vol.  17,  1820.  The 
general  account  of  the  morphology  of  the  families  was  taken  for  the  most  part  from  Anatomie  der 
Dicotyledonen,  H.  Solereder,  Stuttgart,  1899.  Other  authorities,  however,  were  also  referred  to, 
but  they  are  generally  available  and  need  not  be  given  specifically. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


Ill 


There  are  264  species  in  the  Cape  Province.  A  marked  peculiarity 
of  the  African  species,  as  opposed  to  those  in  Australia,  is  that  whereas 
in  the  latter  continent  many  of  the  family  are  to  be  found  in  arid 
regions,  in  southern  Africa  all  of  the  species  are  apparently  restricted 
to  habitats  which  are  humid  or  only  periodically  dry. 

The  anatomical  characters  of  the  large  family  consisting  of  herbs, 
shrubs,  and  trees,  are  various,  but  the  following  respecting  the  leaf  may 
be  mentioned:  The  structure  may  be  isosymmetrical  or  dor  si- ventral. 
Sclerenchyma  may  occur  in  the  mesophyll,  and  there  may  be  hypoderm. 
The  stomata  may  be  superficial  or  deeply  placed.  The  leaves  are 
mostly  reduced  in  size  and  leathery  in  texture. 

Protea  neriifolia,  the  structure  of  which  is  outlined  in  an  earlier 
section,  has  palisade  chlorenchyma,  a  heavy  outer  epidermal,  and 
fairly  deeply  sunken  stomata.  Sclerenchyma  is  present  in  connection 
with  the  conductive  tissue.  The  relatively  small  development  of 
sclerenchyma,  however,  is  a  marked  contrast  to  the  condition  char¬ 
acteristic  of  such  a  species  as  Hakea  leucoptera,  for  example,  of  arid 
South  Australia.1 

Menispermacet®. 

The  Menispermacese  is  largely  tropical  and  is  represented  in  southern 
Africa  by  about  7  species,  of  which  6  occur  in  Natal-Zululand  district 
and  2,  one  being  common  to  the  two  districts,  in  Uitenhage-Port 
Elizabeth.  The  plants  are  undershrubs,  shrubs,  and  climbers. 

The  leaf  is  generally  dorsi-ventral  and  isosymmetral  in  structure, 
whereby  the  chlorenchyma  in  palisades  is  rarely  found.  Mucilaginous 
strata  may  be  organized.  Sclerenchyma  may  be  present  in  the  meso¬ 
phyll.  One-celled  or  two-celled  cover  trichomes  and  multicellular 
secretion  hairs  occur  in  some  species,  and  also  hydathodes,  among 
other  structural  features. 

Short  palisades  were  found  on  both  sides  of  the  leaf  in  Antizoma 
capensis,  and  a  heavy  outer  epidermal  wall,  which,  together  with  the 
relatively  small  leaflets,  are  the  main  xerophytic  characters.  These, 
especially  the  former,  are  apparently  unusual  in  the  family. 

Aizoace^e. 

The  Aizoacese  are  succulents  which  occur  chiefly  in  southern  Africa, 
although  a  few  species  are  to  be  found  in  the  Mediterranean  region, 
Australia,  and  America. 

Of  the  family,  Galenia  africana,  one  of  the  species  studied  in  this 
paper,  and  numerous  species  of  the  genus  Mesembryanthemum,  are 
abundant  in  the  Karroos,  including  the  Upper  and  Lower  Karroo. 
Succulents,  including  many  species  of  the  genus  last  named,  are 

1  Plant  habits  and  habitats  in  the  arid  portions  of  South  Australia.  W.  A.  Cannon.  Carnegie 
Inst.  Wash.  Pub.  No.  308,  1921,  p.  120. 


112 


FEATURES  OF  THE  VEGETATION  OF  THE 


dominant  in  the  southern  portion  of  the  Karroos.  The  diversity  of 
Jorms  is  most  marked. 

Among  the  most  striking  anatomical  features  of  the  family  are  the 
following:  Most  highly  characteristic  is  the  (frequent)  great  differentia¬ 
tion  of  the  epidermis,  in  which  the  development  of  vesicular  cells  with 
water-storage  capacity  is  a  feature.  The  leaf-structure  is  isosym- 
metrical  as  to  the  mesophyll,  in  which  the  entire  mesophyll  may  be 
palisades  or  only  the  subepidermal  layers  (or  modified  palisades). 
There  may  be  a  wax  covering  to  the  epidermis  ( Mesembryanthemum 
sp.)  or  trichomes  by  which  the  rate  of  water-loss  may  be  lowered.  In 
a  few  species  of  Mesembryanthemum  (M.  glaucum,  etc.)  tannin  ducts 
are  to  be  found,  one  or  two  cell-layers  under  the  epidermis.  Calcium 
oxalate  is  present  as  crystals  of  various  types,  of  which  some  are  minute 
and  occur  in  the  membrane  of  the  epidermis  as  determined  first  by 
Solms-Laubach.1  Finally,  it  should  be  mentioned  that  the  organ¬ 
ization  of  mucilages  is  a  characteristic  of  at  least  species  of  Mesembry¬ 
anthemum. 

Galenia  africana  conforms  in  the  epidermal  structures,  in  the  char¬ 
acter  of  the  trichomes,  and  in  the  presence  of  calcium  oxalate,  to  these 
family  features,  but  whether  in  the  metabolic  processes  pentosans,  or 
mucilages,  are  formed  by  the  species  has  not  been  determined.  Galenia 
is  not  a  true  succulent.  The  transpiration  surface  is  greatly  reduced 
and,  as  appears  in  another  connection,  the  form  of  root-system  is  typi¬ 
cal  of  sclerophytes  rather  than  of  species  with  water-balance.  It  is 
not  clear,  therefore,  whether  the  species  exhibits  direct  adjustment  to 
it  environment,  so  far  as  its  morphology  shows  this,  beyond  the  re¬ 
duction  in  the  leaf-surface. 

Capparidaceas. 

The  Capparidacese  are  herbs,  shrubs,  climbers,  and  trees  of  the 
tropics  and  subtropics.  There  are  over  20  species  in  southern 
Africa,  of  which  17  occur  in  Natal  and  6  in  the  Port  Elizabeth-Uiten- 
hage  district,  3  of  these  being  in  both  regions. 

The  anatomical  characters  of  the  leaves  of  the  family  are  extremely 
varied  and  need  not  be  set  forth  in  this  place  (cf.  Solereder,  p.  78). 
Certain  features  which  appear  to  run  through  the  family  may,  how¬ 
ever,  be  referred  to.  For  instance,  sclerenchyma  of  various  types 
is  present  in  the  mesophyll.  Calcium  oxalate  mostly  occurs  as  spher¬ 
ical  masses  or  small  prismatic  crystals.  Two-armed  or  two-branched 
unicellular  trichomes  occur  in  some  Capparis  species,  but  the  kinds  of 
cover  trichomes  are  “endless.” 

In  the  course  of  adjustment  to  arid  conditions,  Cadaba  juncea,  the 
representative  of  the  family  studied,  has  undergone  a  marked  reduc- 

1  Ueber  einige  geformte  Vorkomnisse  oxalsauren  Kalkes  in  Lebenden  Zellmembranen.  Bot. 
Zeit.,  1871. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA.  113 

tion  in  the  foliar  transpiration  surface,  and  has  developed  tissues  with 
heavy  walls,  namely,  outer  epidermal  wall  and  both  fibrous  and  tra- 
cheid-like  sclerenchyma.  The  stomata  of  green  branches  are  deeply 
placed  and  the  outer  vestibule  well  developed.  The  palisade  type  of 
chlorenchyma  is  present. 


Tiliacea:. 

This  great  tropical  genus  is  represented  in  southern  Africa  by  19 
species  or  less. 

Mucilaginous  cells  occur  in  the  epidermis  in  some  species  of  one  sec¬ 
tion  of  the  family.  The  structure  of  the  leaf  may  be  isosymmetrical, 
although  for  most  of  the  species  it  is  dorsi- ventral.  Both  cover  and 
secretion  hairs  occur.  For  other  details  the  reader  is  referred  to 
Solereder,  page  176. 

The  leaf  of  Grewia  cana ,  as  representing  the  Tiliacese,  was  found  to 
have  a  heavy  covering  of  trichomes.  The  epidermis  is  not  heavy  and 
the  outer  wall  is  thin.  Although  in  the  section  to  which  the  genus 
belongs  mucilage  cells  are  not  reported  for  the  epidermis,  the  reaction 
with  chloroiodide  of  zinc  seems  to  show  that  the  epidermal  cells  do 
in  fact  contain  mucilage  to  a  certain  extent,  even  if  there  may  be  no 
especial  mucilaginous  epidermal  cells. 

Anacardiaceai. 

The  Anacardiacese  occur  chiefly  in  warm  countries.  Of  the  family, 
the  genus  Rhus  is  largely  represented  in  southern  Africa,  and  extends 
through  tropical  Africa,  northern  Africa,  the  Mediterranean  region, 
Arabia,  India,  the  Himalayas,  China,  North  America,  and  Mexico, 
according  to  Bews,  who  gives  the  following  information  regarding  the 
distribution  of  the  genus  in  South  Africa:1 

“When  we  look  at  the  present-day  distribution  of  the  species  of  Rhus  in 
South  Africa,  we  find  certain  of  them  *  *  *  very  widespread,  as  pioneer 

species  in  the  xerosere.  Other  *  *  *  occur  along  the  streams  and  are 

important  in  the  hydrosere,  or  *  *  *  occur  on  the  coast  sand-dunes 

in  the  psammosere.  As  succession  advances,  we  find  species  of  Rhus  *  *  * 

dominant  in  climax  vegetation  on  the  dry  doloritic  Karroo  kopjes.  *  *  * 

In  more  mesophytic  forest  areas  we  find  various  steps  in  the  adaptation  to 
more  favorable  conditions  in  a  large  series  of  forms  till  we  reach  such  large 
forest-trees  as  the  red  currant  ( Rh .  Icevigata),  which  grows  fifty  to  eighty 
feet  high,  and  two  to  four  feet  in  stem  diameter.  Rhus  also  shows  adaptation 
to  purely  grassland  conditions.” 

Two  species  of  Rhus  were  studied  in  connection  with  leaf-structure, 
of  which  one,  Rhus  viminalis ,  is  found  in  or  along  the  streamways  of 
the  Karroo,  usually  dry,  and  the  other,  Rhus  sp.,  occurs  on  the  top  of 
kopjes,  at  least  in  the  western  part  of  the  Karroo. 


1  Some  general  principles  of  plant  distribution,  etc.,  1.  c.,  p.  19. 


114 


FEATURES  OF  THE  VEGETATION  OF  THE 


Among  the  leading  structural  features  of  the  leaf  of  the  family  are 
the  following:  Resin  channels  are  present  and  simple  one-celled  tri- 
chomes  and  glandular  trichomes  of  the  greatest  variety  of  form,  which 
in  certain  species  may  secrete  a  lacquer  covering  to  the  leaf.  Scler- 
enchyma  is  given  as  a  character  of  the  (primary)  cortex,  but  is  not 
included  by  Solereder  among  the  anatomical  features  of  the  leaf. 

Inasmuch  as  the  leading  points  of  foliar  structure  for  the  species 
of  Rhus  studied  has  been  given  in  an  earlier  paragraph,  it  will  only 
be  necessary  in  the  present  connection  to  call  attention  to  the  follow¬ 
ing:  In  both  species  examined  the  structure  is  dorsi-ventral,  but 
the  palisades  are  relatively  much  longer  in  Rhus  viminalis  than  in 
Rhus  sp.  Sclerenchyma  is  developed  in  connection  with  the  veins  in 
both  species,  but  is  more  pronounced  in  the  former  than  in  the  latter. 
In  neither  species  is  the  outer  epidermal  wall  especially  heavy.  A 
heavy  resinous  (?)  coating  occurs  on  the  dorsal  surface  of  Rhus  sp. 
This  feature  was  not  observed  in  Rhus  viminalis. 

CELASTRACEiE. 

The  Celastracese  is  a  rather  small  family  of  widely  scattered  species 
of  shrubs,  lianes,  and  trees,  being  found  in  the  tropics  and  subtropics 
and  temperate  zones  of  both  hemispheres,  and  in  all  continents,  in¬ 
cluding  Australia.  Of  the  family  about  47  species  occur  in  southern 
Africa,  approximately  one-half  of  which  are  in  the  Natal  region. 

Among  the  structural  features  which  occur  in  the  family  are  the 
following:  Mucilaginous  epidermal  walls,  calcium  oxalate  as  spherical 
crystal  masses  or  single  crystals;  hypoderm;  resin  cells  in  epidermis 
and  mesophyll;  sclerenchyma  not  given  for  leaf,  but  present  as  fibers 
in  cortex  of  stem. 

In  Gymnosporia  huxifolia  leaf  the  outer  epidermal  wall  is  heavy, 
the  guard-cells  of  the  stomata  are  somewhat  raised  above  the  general 
leaf-surface,  a  hypoderm  is  present,  and  sclerenchyma  occurs  in  asso¬ 
ciation  with  the  conductive  tissue.  The  marked  development  of 
spines,  morphologically  branches,  and  reduction  in  size  of  the  leaves, 
are  further  indications  of  an  adjustment  to  arid  environment.  It  is 
perhaps  worthy  of  note  that  the  chlorenchyma  is  not  of  true  palisades 
and  such  are  not  reported  by  Solereder  for  other  species  of  the  family. 

ARALIACEiE. 

The  Araliacese  are  chiefly  tropical.  About  7  species  are  found  in 
southern  Africa,  all  in  Natal,  and  but  2  in  the  Uitenhage-Port  Eliza¬ 
beth  district. 

The  anatomical  features  of  the  leaves  are  various  and  include 
hypoderm  on  the  dorsal  side,  secretory  ducts,  extremely  variable 
glandular  and  cover  trichomes,  secreting-cells  of  calcium  oxalate, 
dorsi-ventral  symmetry,  etc- 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


115 


In  Cussonia  spicata  the  hypoderm  is  the  most  striking  anatomical 
feature,  although  the  outer  epidermal  walls  of  both  surfaces  are  fairly 
well  developed.  It  should  be  noted  that  Solereder  (7.  c.,  p.  483)  cites 
22  species  in  which  a  hypodermal  development  occurs.  Of  these,  only 
2  species  are  given  by  Marloth  (Das  Kapland)  for  South  Africa,  both 
of  which  are  of  the  genus  Cussonia.  Secretory  ducts  occur  in  con¬ 
nection  with  the  conductive  tissue.  There  is  little  sclerenchyma. 
There  appears,  therefore,  to  be  little  in  the  foliar  tissues  to  suggest 
structural  development  in  relation  or  adjustment  to  a  severely  arid 
environment,  which  accords  with  the  known  distribution  of  the  species 
in  which  such  conditions  are  apparently  avoided. 

Leguminosze. 

This  large  family  is  well  represented  in  southern  Africa,  283  species 
being  reported  from  the  Natal-Zululand  district  and  149  from  the 
Uitenhage-Port  Elizabeth  district.  Species  of  the  Leguminosse  are 
to  be  found  very  generally  distributed  throughout  the  subcontinent. 
Of  the  family,  Sutherlandia  frutescens  is  given  by  Bews  as  an  example 
of  an  isolated  species  with  no  obvious  connections,1  but  apparently  very 
widely  distributed,  and  Bauhinia  marlothii  is  of  small  genus  and  the 
species  may  be  limited  to  the  arid  Namaqualand  district.  The  former 
is  of  the  subfamily  Papillionatse  and  the  latter  of  the  Caesalpinse. 

Papillionatse. 

The  anatomical  features  of  the  subfamily  are  so  varied  that  no 
attempt  can  be  made  in  this  place  to  summarize  them.  However,  a 
few  salient  characters  can  be  pointed  out,  as  follows:  Among  the 
features  common  to  the  subfamily  are  the  failure  of  spherical  crystals 
(aggregates),  and  that  of  common  unicellular  trichomes,  and,  of 
positive  characters,  the  occurrence  of  rod-shaped  crystals  and  of 
elongated,  tubular  cells  containing  tannin  or  albuminous  substances. 
Thus  the  want  of  common  structural  features,  with  the  correlation, 
great  variation  in  structure,  are  noteworthy  characteristics.  For 
details  the  reader  is  referred  to  the  account  by  Solereder  and  the  refer¬ 
ences  there  cited. 

It  will  be  seen  by  referring  to  the  sketch  of  some  of  the  leading 
anatomical  features  of  the  leaves  of  Sutherlandia  frutescens,  as  given 
in  an  earlier  section,  that  the  development  of  palisades  on  both  sides 
of  the  leaf  and  the  presence  of  a  heavy  outer  epidermal  wall,  bespeak 
the  expected  structural  adjustment  to  an  arid  environment,  but  the 
apparent  absence  of  sclerenchyma  may  be  of  phylogenetic  significance. 
Not  enough  is  known,  however,  regarding  the  leaf  anatomy,  either 
of  the  species  in  question  or  of  those  most  nearly  related  to  it,  to  con¬ 
sider  the  subject  further  at  the  present  time. 

1  Some  general  principles  of  plant  distribution  as  illustrated  by  the  South  African  flora.  Ann. 
Bot.,  vol.  35,  p.  19,  1921. 


116 


FEATURES  OF  THE  VEGETATION  OF  THE 


CiESALPINvE. 

There  are  few  anatomical  characters  of  the  leaf  held  in  common  by 
the  subfamily  Caesalpinae,  but  among  such  are  the  occurrence  of 
calcium-oxalate  crystals  in  spherical  masses  and  the  formation  of 
simple,  unicellular  trichomes,  although  the  last-mentioned  feature 
is  not  without  exceptions.  For  greater  detail  the  reader  is  again 
referred  to  the  summary  by  Solereder  (p.  319),  but  the  following  points 
can  be  mentioned:  The  epidermis  is  mostly  of  one  cell-layer,  the 
chlorenchyma  of  some  species  is  of  palisades,  in  some  species  the 
epidermis  is  subpapillose  and  in  other  papillose.  The  stomata  are 
mostly  on  the  ventral  surface,  and  the  guard-cells  usually  on  a  level 
with  the  leaf-surface.  Sclerenchyma  of  various  types  may  be  present. 

As  to  the  foliar  structure  of  Bauhinia  marlothii,  which  has  already 
been  outlined,  it  is  sufficient  for  the  present  purpose  to  point  out  that 
the  fairly  large  leaflets  have  chlorenchyma  of  palisades  on  both  faces, 
that  trichomes  are  wanting,  as  also  is  sclerenchyma,  and  that  the  epi¬ 
dermal  cells  in  age  become  papillose,  or  at  least  subpapillose.  In  this 
case  the  outer  epidermal  wall  is  thin,  and,  as  occurs  when  there  is  a 
heavy  cover  of  trichomes,  the  stomata  are  superficially  situated. 

Crassulacea;. 

The  Crassulaceae,  which  are  herbs  or  undershrubs,  are  to  be  found 
in  warm  temperate  regions,  about  one-half  being  in  southern  Africa. 
But  some  occur  in  Australia,  southern  Asia,  and  central  subtropical 
America,  the  Mediterranean  region,  etc.  There  are  about  108  species 
in  southern  Africa,  about  one-half  of  which  occur  in  Natal  and  Zulu- 
land.  Of  the  genus  Crassula,  34  species  have  been  reported  from  the 
Central  Provinces,  the  Karroos,  and  14  from  the  arid  region  northwest 
of  the  latter. 

The  following  are  some  of  the  leading  structural  characteristics 
of  the  leaves  or  chlorophyll-bearing  organs  of  the  family.  True 
palisade  cells  are  seldom  formed.  Stomata  usually  are  on  both  leaf- 
surfaces.  A  covering  of  trichomes  is  unusual.  Calcium  oxalate  may 
be  formed  as  single  crystals,  spherical  masses,  or  fine  crystals;  in 
the  latter  event  they  may  either  be  in  the  lumen  of  the  cells  or  em¬ 
bedded  in  the  walls.  A  waxy  covering  of  the  epidermis  sometimes 
occurs.  But  the  most  striking  feature  of  the  family  as  a  whole  is  the 
presence  of  mucilages  and  the  general  succulent  habit,  extending  to  the 
leaves,  which,  in  some  species,  are  strongly  developed,  in  which  case 
the  forms  are  so-called  leaf  succulents. 

The  leaf  of  Cotyledon  paniculata,  which,  although  fleshy,  is  deciduous, 
conforms  in  the  main  features  of  its  structure  to  the  general  structural 
relations  of  the  family.  These  have  already  been  sketched.  It  is 
sufficient  to  point  out  here  that  heavy-walled  supporting  tissue  is 
absent  and  that  the  leaf  is  highly  mucilaginous.  Associated  possibly 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


117 


with  the  fact  that  the  leaf  is  fugacious,  the  outer  “protective”  covering 
and  structural  arrangements  of  whatever  kind  of  the  leaf,  do  not 
appear  to  be  suitable  to  withstand  successfully  severe  dryness  of  the 
atmosphere.  The  species  is  a  stem  succulent. 

Labiate. 

The  large  family  of  the  Labia tse  is  largely  found  in  the  Northern 
Hemisphere,  and  especially  in  dry  and  sunny  situations  which  appear 
to  be  favorable  for  the  formation  of  the  ethereal  oils  characteristic  of 
it.  In  southern  Africa  it  is  largely  represented  in  the  Natal-Zululand 
districts,  in  which  98  species  occur.  In  the  region  about  Uitenhage 
and  Port  Elizabeth  are  42  species,  of  which  15  are  common  to  the  two 
floral  districts.  So  far  as  concerns  the  genus  Stachys,  which  has  been 
examined  in  the  present  study,  21  species  are  in  the  region  first  named 
and  9  are  in  the  one  farther  southwest;  4  species  are  common  to  the 
two.  The  Labiatse  do  not  appear  to  be  found  to  any  extent,  if  at  all, 
in  the  semi-desertic  regions  of  the  subcontinent. 

In  a  family  so  large,  consisting  of  over  2,700  species,  the  foliar 
structures  are  necessarily  various,  even  if  the  family  as  a  whole  is  one 
of  the  most  natural  plant  groups.  For  greater  details  the  reader  is 
referred  to  Solereder  (p.  718)  and  to  the  authors  there  cited.  It  will 
be  sufficient  for  the  immediate  purposes  to  point  to  the  following 
structural  features:  Secretory  trichomes,  from  which  the  ethereal 
oils  are  derived,  are  of  variable  structure,  as  also  are  the  cover  hairs. 
Both  may  be  extensively  developed  and  form  special  characteristics. 
Hypoderm  is  present  in  certain  species,  and  resiniferous  cells  as  well. 

So  far  as  regards  the  structure  of  the  leaf  of  Stachys  sp.,  an  outline 
of  which  is  given  above,  it  need  only  be  mentioned  that  there  are  no 
apparent  structural  features  which  point  to  the  immediate  influence  of 
an  arid  environment,  unless  it  is  the  formation  of  a  heavy  covering 
of  trichomes,  and  this,  as  will  be  seen  from  the  above  sketch  of  family 
structural  characteristics,  may  be  regarded  as  being  only  a  possible 
accentuation  of  tendencies  already  possessed  by  other  species  of  the 
family. 

SCROPHULARIACEiE. 

The  Scrophulariacese  is  a  cosmopolitan  family  occurring  mostly  in 
temperate  regions.  According  to  Bews,  there  are  21  genera,  widely 
scattered  over  South  Africa,  of  which  3  are  in  the  Kalahari  region. 
“Most  of  the  genera  afford  examples  of  widespread  species  giving  rise 
to  rare  endemics.”  1 

Solereder  gives  an  extensive  list  of  structural  peculiarities  of  the 
leaf,  to  which  the  reader  is  referred.  These  include  great  variety  of 
cover  and  secretion  hairs;  calcium  oxalate  seldom  occurs  and  then  in 

1  Some  general  principles  of  plant  distribution  as  illustrated  by  the  South  African  flora.  J.  W. 
Bews.  Ann.  Bot.,  vol.  37,  p.  24,  1921. 


118 


FEATURES  OF  THE  VEGETATION  OF  THE 


small  crystals,  rarely  in  large  crystals  or  in  crystal  aggregates.  Crys¬ 
tals  of  carotin  and  protein  crystals  frequently  are  to  be  found  in  the 
mesophyll.  The  leaf-structure  is  isosymmetrical  or  dorsi-ventral. 
In  desert  species  the  epidermal  cells  have  heavy  walls  and  carry 
tannin. 

So  far  as  concerns  the  structure  of  Aptosium  indivisum,  which  was 
studied  and  the  leaf-structure  of  which  was  sketched  in  an  earlier 
paragraph,  it  is  sufficient  to  remark  that  the  leading  features  of  the 
structure  are  the  very  heavy  outer  epidermal  wall  and  the  palisade 
chlorenchyma,  and  are  very  evidently  adjustments  to  the  environment. 

AsCLEPIADACEiE. 

The  Asclepiadacese,  which  is  mainly  tropical,  is  largely  represented  in 
southern  Africa  and  chiefly  in  the  Zululand-Natal  district,  where 
156  are  reported;  65  species  occur  in  the  Uitenhage-Port  Elizabeth 
district. 

The  anatomical  features  of  the  family,  as  well  as  the  gross  morphol¬ 
ogy,  are  strongly  diverse.  It  will  be  sufficient  in  this  place  to  point 
to  the  following  relative  to  the  leaves:  The  structure  may  be  dorsi- 
ventral,  or  the  mesophyll  may  be  alike  on  the  two  sides.  Hypoderm 
may  be  present.  Sclerenchymatous  fibers  as  well  as  latex  tubes  run 
irregularly  in  the  mesophyll.  Some  species  have  succulent  leaves. 
In  species  from  the  arid  regions  the  cuticula  is  highly  developed,  and 
there  is  secretion  of  wax.  Many  species  are  stem  succulents,  which 
connotes  the  organization  of  mucilages,  and  hence  the  possession  of  a 
distinct  type  of  metabolism. 

Asclepias  ftliformis  (?),  a  sclerophyllous  species,  as  has  been  de¬ 
scribed  in  the  preceding  section,  possesses  certain  structural  features 
which  conform  to  those  known  to  be  characteristic  of  the  family  as  a 
whole  and  which  need  not  be  repeated  here;  but  in  certain  regards,  also, 
it  is  evident  that  there  is  a  response  to  an  arid  environment,  as  evi¬ 
denced  by  the  character  of  the  tissues  as  a  whole.  Thus,  there  is  a 
relatively  large  development  of  cell-walls,  and  especially  of  the  outer 
epidermal  wall,  and  a  fairly  large  proportion  of  non-living  material. 

ApOCYNACEiE. 

The  Apocynacese  are  chiefly  tropical  and  are  comparatively  rare 
in  extra- tropical  hot  and  temperate  countries.  There  are  about  22 
species  in  southern  Africa,  of  which  18  are  listed  as  occurring  in  the 
Zululand-Natal  district;  1  species  occurs  in  the  Karroo  region. 

The  structure  of  the  leaf  of  members  of  this  family  as  outlined  by 
Solereder,  to  whom  the  reader  is  referred,  is  exceedingly  varied. 
The  leaves  may  be  dorsi-ventral,  or  the  structure  may  be  similar  on 
both  leaf-surfaces.  There  may  be  hypoderm,  and  cover  hairs  of 
various  forms  are  to  be  found.  The  stomata  are  of  different  types. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


119 


The  cell-walls  in  the  spongy  tissue  may  be  mucilaginous,  and,  finally, 
there  may  be  palisade  sclerenchyma. 

As  to  the  special  structural  features  of  Carissa,  which  are  men¬ 
tioned  more  fully  above,  it  need  only  be  pointed  out  that  there  is  a 
marked  reduction  of  the  transpiration  surface,  a  heavy  outer  epidermal 
wall  is  formed,  and  sclerenchyma  occurs  in  association  with  the  con¬ 
ductive  tissue.  It  is  not  impossible  that  all  of  the  leading  morpho¬ 
logical  characters  of  the  species,  except  possibly  the  heavy  outer  epi¬ 
dermal  wall  and  the  reduced  surface,  can  be  duplicated  in  other 
members  of  the  family  which  occur  in  less  arid  habitats. 

Ebenaceaj. 

Species  of  Ebenaceae  occur  in  tropical  and  subtropical  regions  of 
Asia,  South  Africa,  Australia,  and  America;  they  are  rare  in  the  Medi¬ 
terranean  region.  In  southern  Africa  the  family  is  largely  represented 
in  the  south  and  southeast,  but  species  of  Royena  and  Euclea  occur  in 
the  arid  central  province,  the  Karroos,  as  well. 

The  structure  of  the  leaf  is  generally  dor  si- ventral.  Hypoderm  is 
formed  in  some  species.  Sclereids  are  sometimes  to  be  found,  but 
seldom  in  the  mesophyll.  Cover  hairs,  of  simple  structure,  are  char¬ 
acteristic,  but  two-armed  trichomes  and  glandular  trichomes  are  also 
formed.  Mucilages  are  not  organized. 

Euclea  undulata  and  Royena  pallens  were  examined.  Both  species 
agree  in  having  trichomes,  especially  in  the  young  leaf.  Where  they 
persist  the  outer  epidermal  wall  is  light,  otherwise  it  is  heavy.  The 
stomata  are  not  sunken.  In  Euclea  the  structural  symmetry  is  dorsi- 
ventral  and  sclerenchyma  occurs  with  the  conductive  tissue.  In 
Royena  the  chlorenchyma  is  palisade  and  of  modified  palisade,  with 
the  effect  that  the  symmetry  is  modified  accordingly.  There  appears 
to  be  no  sclerenchyma  in  this  species. 

Apparently  neither  of  the  species  mentioned  has  an  extreme  type 
of  xerophytic  leaf-structure  and  also  apparently  neither  occurs  in  the 
most  arid  situations. 

Composite. 

In  South  Africa  over  40  genera  of  the  Composite  are  of  general 
distribution,  over  40  genera  mainly  in  the  southwest,  a  small  group 
mainly  central,  about  20  genera  from  the  Transvaal  and  Natal  along 
the  coast  of  the  south,  and  a  few  with  restricted  distribution.  Of  the 
genera  whose  leaf  anatomy  is  sketched  in  this  paper,  Pteronia  is 
strongly  represented  in  the  Karroo  provinces  and  in  the  arid  region  to 
the  northwest,  but  apparently  is  not  reported  from  Natal,  but  occurs 
in  the  Port  Elizabeth  district.  Euryops  is  also  represented  in  these  two 
arid  regions,  but  occurs  also  in  Natal,  as  well  as  in  Port  Elizabeth 
district,  and  the  same  can  be  said  of  Pentzia. 


120 


FEATURES  OF  THE  VEGETATION  OF  THE 


Owing  to  the  great  number  of  species  of  this  family  and  the  complex¬ 
ity  of  the  life-forms,  and  that  of  the  structure  of  the  foliar  organs,  it  is 
not  practicable  in  this  place  to  present  an  outline  of  the  anatomical 
characteristics,  particularly  of  those  representatives  of  the  family 
which  live  in  the  more  humid  region  of  the  subcontinent,  of  which  the 
anatomy  may  not  have  been  worked  out.  Reference  to  the  outline 
of  the  structure  of  the  leaves  of  the  genera  represented  in  the  study 
in  a  foregoing  section  will  give  some  idea  of  the  structural  adjustment 
to  such  conditions  of  aridity  as  obtain  where  they  were  observed  in  the 
field.  It  will  be  seen  that  there  are  in  general  such  structural  features 
as  may  be  expected  under  such  conditions,  including  a  heavily  devel¬ 
oped  outer  epidermal  wall,  which  may  lead  almost  to  the  total  obliter¬ 
ation  of  the  lumen  of  the  cells,  and  generally  these  features  include 
marked  development  of  sclerenchyma  as  well.  There  is  also  in 
certain  of  the  species,  and  particularly  when  young,  trichomes  of 
various  kinds,  and  in  certain  of  them  secretory  ducts.  Such  charac¬ 
ters,  with  a  reduction  in  the  transpiration  surface,  possibly  would 
separate  these  species  from  the  related  species  of  the  more  humid 
regions,  whatever  the  structure  otherwise  of  the  latter  might  be. 
But  as  to  the  more  exact  possible  derivation  of  structures  in  these 
xerophytes,  it  seems  at  the  present  impossible  to  speak. 

SOME  CONCLUSIONS. 

In  considering  the  relation  and  possible  origin  of  the  foliar  struc¬ 
tures  dealt  with  in  this  section,  it  is  desirable  to  call  to  the  attention 
the  following  points,  which,  for  the  sake  of  clearness  and  brevity,  will 
be  stated  didactically : 

(1)  As  the  summaries  of  the  distribution  of  the  families  indicate, 
it  is  apparent  that  the  family  of  each  species  used  is  also  in  part  to 
be  found  under  favorable  conditions,  so  far  as  the  rainfall  is  concerned. 
This  is  not  an  unusual  condition,  since,  according  to  Bews,1  “the 
examples  of  xerophytic  shrubby  species  being  closely  allied  to  the 
mesophytic  forest  species  are  *  *  *  very  numerous.” 

(2)  The  brief  statements  of  the  main  foliar  structural  features  of  the 
families  compared  to  the  summaries  of  the  same  characteristics  for 
each  species  indicate  that  often,  if  possibly  not  always,  the  definite 
family  characters  may  be  clearly  recognized  in  the  xerophytic  relative. 
The  characteristic  structures  of  the  xerophilous  species  are,  thus,  in 
part  a  modification  of  family  structures  by  reason  of  which,  at  least 
as  indicated  by  the  morphology  of  the  leaves  alone,  survival  is  accom¬ 
plished. 

(3)  The  list  at  top  of  page  21  partially  summarizes  the  observed 
xerophytic  structures  and  suggests  the  family  ones  of  which  they  may 
be  a  modification. 

1Some  general  principles  of  plant  distribution  as  illustrated  by  the  South  African  flora,  Ann. 
Bot.,  vol.  35,  p.  31,  1921. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


121 


MESOPHYTE8. 

Leaves  various,  often  of  large  size  and 
relatively  thin. 

Trichomes  often  present,  at  least  in 
young  leaves ;  cover  and  glandular  tri¬ 
chomes,  the  latter  of  which  may  secrete 
various  substances,  including  those  which 
are  resinous,  and  ethereal  oils  as  well. 

Outer  thin  epidermal  wall,  lightly  cutic- 
ularized,  may  be  covered  lightly  with  wax 
of  epidermal  origin. 

Stomata,  superficially  placed,  provided 
with  outer  vestibule  ridge.  Portion  of 
guard-cell  walls  may  be  cuticularized. 


Structural  symmetry  usually  dorsi-ven- 
tral.  Chlorenchyma  cuboid  or  palisade. 

Sclerenchyma  may  be  present;  if  so, 
usually  in  association  with  fibro-vascular 
bundles,  but  not  strongly  developed. 

Metabolism  leads  to  the  formation  of  a 
relatively  small  amount  of  cell-wall  material, 
and  but  little  material  that  is  mucilaginous. 


XEROPHYTES.1 

Leaves  usually  small,  or  wanting,  and 
never  thin,  sometimes  succulent. 

Trichomes  may  be  persistent.  Cover 
trichomes  may  be  associated  with  marked 
xerophytic  type  of  epidermis.  Glandular 
trichomes  may  secrete  heavy  outer  water¬ 
proof  coating  of  leaf,  or  may  organize 
ethereal  oils. 

Outer  epidermal  wall  may  be  very  thick, 
may  be  heavily  cuticularized,  and  may  give 
rise  to  an  outer  cover  of  wax. 

Stomata  may  be  superficially  or  deeply 
placed;  if  the  latter,  the  pit  may  be  con¬ 
stricted  at  entrance  as  well  as  by  other 
vestibule  ridges.  Portion  of  guard-cell 
walls  may  be  cuticularized,  and  those  of 
the  pit  as  well. 

Structure  either  isosymmetrical  or  dorsi- 
ventral.  In  the  former  chlorenchyma  of 
palisades,  or  in  succulents  cuboid  or  modi¬ 
fied  palisades. 

Sclerenchyma  may  be  a  marked  struc¬ 
tural  feature;  usually  in  connection  with 
conductive  tissue. 

Under  arid  conditions  the  polysac¬ 
charides  may  be  largely  converted  into 
anhydrides  or  wall  material,  or,  in  less  arid 
conditions,  into  pentosans  or  mucilages 
(succulents). 


1  Unmodified  or  slightly  modified  family  characteristics  are  not  included  in  the  list. 


122 


FEATURES  OF  THE  VEGETATION  OF  THE 


OBSERVATIONS  ON  THE  FOLIAR  TRANSPIRING 
POWER  IN  WINTER  AND  SPRING. 

The  studies  on  the  transpiring  power  of  some  perennials  of  southern 
Africa  were  carried  out  during  the  cool  season,  between  the  last  of 
June  and  the  last  of  October,  on  species  in  the  Namib  Desert,  and 
especially  in  the  Central  Karroo. 

The  Stahl-Livingston  cobalt-chloride  method  was  used,  with  certain 
minor  changes  to  adapt  it  to  the  conditions  met  in  the  field  work. 
Thus,  in  order  to  dry  the  hygrometric  paper,  use  was  made  of  a  folding 
photographic  dark-room  lantern  having  a  metal  top.  The  heat  was 
obtained  by  the  use  of  short  candles,  “night-light,”  which  are  con¬ 
tained  in  flat  tin  boxes.  These  burn  with  a  fairly  even  flame  and  do  not 
greatly  decrease  in  height  as  they  are  consumed.  The  cobalt-chloride 
paper  was  cut  into  appropriate  sizes  and  kept  in  small  and  tight  tin 
boxes.  In  the  field  the  boxes  were  placed  on  top  of  the  little  stove. 
The  hygrometric  papers  were  in  this  way  properly  dried  and  were  kept 
suitably  dry  as  long  as  was  required.  In  no  case  was  it  possible  to  dry 
the  papers  over  the  candle  flame  directly,  owing  to  the  almost  con¬ 
stant  wind,  which  made  it  necessary  to  use  a  small  screen  even  with  the 
heating  apparatus  described. 

Glass  clips  with  color  standards  and  holding  cobalt-chloride  paper 
were  placed  as  controls  near  the  leaves  studied.  This  was  found  to  be 
advisable,  owing  to  the  long  reaction-time  which  many  of  the  leaves 
were  found  to  have. 

In  the  event  that  the  leaves  were  too  thick  to  use  with  the  glass 
clip  supplied,  a  substitute  was  found  by  using  large  metal  clips  with 
celluloid  in  place  of  the  glass. 

It  was  found  necessary  to  add  to  the  glass  clip  with  its  rather  delicate 
spring  an  additional  spring  clip  in  order  to  get  firmer  attachment  to 
the  leaf.  This  was  especially  necessary  in  the  case  of  small  leaves. 
For  the  purpose  a  wooden  clothes-pin  with  spring  was  found  to  be 
suitable. 

In  running  a  series  of  tests,  several  clips  were  used  at  one  time,  and 
whenever  possible  also  the  same  leaf  and  even  the  same  portion  of  the 
leaf  was  studied  repeatedly.  In  this  way  the  course  of  the  transpiring 
power  of  the  plant  could  better  be  determined. 

As  is  essential  for  the  work,  the  temperature  of  the  air  near  the 
leaves  being  tested  was  recorded,  and  such  temperatures  used  in 
calculating  the  indices  of  the  transpiring  power  in  the  way  worked 
out  for  the  method.1 

But  few  attempts  were  made  to  run  the  tests  during  the  hours  of 
darkness,  in  part  because  it  was  found  to  be  difficult,  if  not  impossible, 
even  with  the  aid  of  a  flash-light,  to  surely  distinguish  the  color  reac- 

1  Improvements  in  the  method  for  determining  the  transpiring  power  of  plant  surfaces  by 
hygrometric  paper.  B.  E.  Livingston  and  Edith  B.  Shreve.  Plant  World,  vol.  19,  p.  287,  1918. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


123 


tions.  Accordingly,  the  daily  course  of  the  observations  was  from 
about  sunrise  to  about  sunset,  when  the  tests  were  made  at  approxi¬ 
mately  2-hour  intervals. 

The  transpiration  studies  were  carried  out  on  the  following  species : 
At  Beaufort  West:  Aloe  schlechteri ,  Gasteria  disticha,  Grewia  cana, 
Gymnosporia  buxifolia,  and  Massonia  latifolia.  At  Matjesfontein: 
Aloe  striata ,  Cotyledon  coruscans,  C.  paniculata,  Eucalyptus  globulus 
(?),  Euclea  undulata,  Euryops  lateriflorus,  Protea  neriifolia  (Tweedside), 
Rhus  viminalis,  and  Rhus  sp.  (Whitehill).  Namib  Desert:  Bauhmia 
marlothii ,  Welwitschia  mirabilis. 

Beaufort  West. 

Aloe  schlechteri. 

At  Beaufort  West  Aloe  schlechteri  (plate  8a)  occurs  on  the  north¬ 
east  upper  slopes  of  a  chain  of  low  kopjes  which  are  east  and  north  of 
that  place.  Only  two  groups  of  a  few  plants  each  were  seen.  Of 
these,  the  ones  especially  studied  were  growing  on  a  kopje  about  2 
miles  to  the  north  of  town.  The  observations  were  made  on  August 
15  and  16,  at  a  time  when  the  leaves  were  turgid  and  when  the  species 
was  about  to  flower. 

On  August  15  the  shade  temperature  of  the  air  near  the  Aloe  selected 
for  study  was  between  15°  C.  in  early  morning  and  24°  C.  about  mid¬ 
day.  The  day  was  fair  and  dry.  There  was  a  strong  breeze  from  the 
north,  which  arose  about  sunrise  and  continued  throughout  the  day. 
The  cobalt-chloride  control  did  not  change  in  color,  and  strips  of  the 
paper  freely  exposed  to  the  air  changed  only  to  light  blue.  Three 
leaves  were  used.  The  indices  of  transpiring  power,  together  with  the 
time  of  the  observations  and  the  reaction-time,  are  given  in  table  13, 
for  leaf  No.  2,  and  illustrate  those  obtained  for  the  two  other  leaves. 

Table  13. — Transpiring  power ,  leaf  No.  2 ,  Aloe  schlechteri,  August  15. 


Reaction-time, 
in  seconds. 

Index  of  trans¬ 
piring  power. 

7h  37m . 

300  to  330 

0.095 

9  7  . 

300  to  420 

.065 

11  31  . 

510 

.030 

5  19  . 

840 

.019 

On  the  following  day  the  experiments  were  run  during  the  morning 
only.  The  day  was  fair  and  a  light  breeze  which  lasted  all  of  the 
forenoon  came  from  southerly  points.  The  humidity  of  the  air  was 
apparently  somewhat  high,  because,  upon  being  exposed  to  the  air, 
the  dark-blue  cobalt-chloride  papers  quickly  changed  to  light  blue 
and  to  pink.  The  temperature  of  the  air  ranged  between  5°  and  22°  C. 
Table  14  summarizes  the  August  16  test  on  leaf  No.  2. 


124 


FEATURES  OF  THE  VEGETATION  OF  THE 


On  August  16,  three  leaves  were  used,  and  it  was  found  that, 
especially  in  early  morning,  there  was  much  variation  in  the  readings. 
On  two  leaves  two  clips  were  read  at  the  same  time  with  widely  dif¬ 
ferent  results  which  were  not  accounted  for.  But  the  lowest  trans- 


Table  14. — Transpiring  power,  leaf  No.  2,  Aloe  schlechteri,  August  16. 


Reaction-time, 
in  seconds. 

Index  of  trans¬ 
piring  power. 

7h  55m . 

180  to  330 

0.198 

9  27  . 

210 

.169 

9  48  . 

360 

.092 

11  9  . 

620  to  720 

.033 

piring  power  of  leaf  No.  2  on  this  day,  including  the  early  reading, 
was  very  much  greater  than  the  highest  for  the  preceding  day.  The 
indices  at  the  late  morning  hours  for  the  two  days  are,  however,  about 
the  same.  Inasmuch  as  the  humidity  was  higher  on  the  second  day 
of  the  observations,  and  for  the  entire  morning,  it  was  not  considered 
probable  that  the  difference  noted  was  owing  to  its  direct  effect  on  the 
cobalt-chloride  paper  by  leakage  around  the  clip,  for  the  reason  that, 
as  above  indicated,  the  later  hours,  when  the  humidity  was  yet  high, 
the  index  of  transpiring  power  for  the  two  days  was  substantially  the 
same.  It  was  consequently  concluded  that  the  variation  in  the  index 
as  between  the  two  days  was  due  to  actual  differences  in  the  transpiring 
power  of  the  plant.  This  makes  the  considerable  range  of  10  to  1  in 
the  index,  which,  however,  should  not  be  considered  the  limits  of 
variation,  as  a  lower  index  would  be  expected  in  a  more  arid  season. 

Gasteria  disticha. 

Gasteria  disticha  (plate  9)  occurs  at  Beaufort  West  on  the  southerly 
upper  slopes  of  the  kopjes  near  town.  Usually  it  is  in  association 
with  some  large  species,  as  Lycium  sp.,  by  which  it  may  be  “protected” 
in  some  fashion.  It  is  a  leaf  succulent. 


Table  15. — Transpiring  power  of  Gasteria  disticha. 


Reaction-time, 
in  seconds. 

Index  of  trans¬ 
piring  power. 

Smaller 

leaf. 

Larger. 

leaf. 

Smaller 

leaf. 

Larger 

leaf. 

*jh  49m 

660 

660 

0.070 

0.070 

9 

7  . 

660 

600 

.047 

.052 

9 

28  . 

1,020 

900 

.030 

.034 

11 

24  . 

1,080 

1,040 

.022 

.023 

12 

13  . 

1,020 

1,040 

.019 

.013 

3 

31  . 

1,740 

1,980 

.010 

.009 

MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


125 


The  transpiration  power  of  Gasteria  was  studied  on  August  19 
and  22.  On  the  first  day  a  reaction-time  between  300  seconds  in 
early  morning  and  3,360  seconds  in  mid-afternoon  was  obtained,  with 
an  index  varying  between  0.097  and  0.009.  On  the  following  day 
observations  were  made  on  two  leaves,  a  larger  and  a  smaller  one, 
which  were  assumed  to  be  of  unlike  age.  The  smaller  leaf  wras  very 
evidently  of  the  preceding  year  and  the  larger  one  may  have  been  sev¬ 
eral  years  old.  Table  15  summarizes  the  leading  results  of  August  22. 

It  will  be  seen,  therefore,  that  the  transpiring  power  of  the  larger 
(older)  leaf  and  of  the  smaller  (younger)  one  is  about  the  same.  There 
seems,  however,  to  be  a  slight  difference  in  that  in  mid-afternooon  the 
smaller  leaf  has  a  relatively  small  index  and  at  mid-day  the  opposite 
is  the  case.  So  far  as  the  latter  condition  is  concerned,  which  is  not 
well  indicated  by  the  table,  the  reaction  of  the  larger  leaf  at  12h  13m 
was  not  wholly  complete  when  the  reading  was  made.  At  3h  30m, 
however,  the  paper  had  changed  to  pink.  In  any  event,  the  index 
of  transpiring  power  of  the  species  was  found  to  be  low. 

Grewia  Cana. 

Grewia  cana  (plate  7b)  is  a  sclerophyllous  shrub  with  evergreen 
leaves  about  1  by  2  cm.  in  size,  which  at  Beaufort  West  occurs  spar¬ 
ingly  on  kopjes,  especially  on  the  southern  slopes.  The  leaves  are  dis¬ 
tinctly  dorsi-ventral. 

The  transpiring  power  of  the  species  was  studied  August  20,  22, 
and  23. 

On  August  20  the  test  was  for  the  purpose  of  determining  whether 
the  two-leaf  surfaces  had  unlike  indices.  The  results  for  that  day 
are  summarized  in  table  16,  which  gives  the  time  of  the  observations 
and  the  indices. 

Table  16. — Transpiring  power  of  dorsal  and  ventral  leaf-surfaces  of 

Grewia  cana,  August  20. 


Dorsal  leaf- 
surface. 

Ventral  leaf- 
surface. 

8h  4m . 

0.020 

0.118 

9  14  . 

.035 

.374 

2  50  . 

.234 

It,  therefore,  is  clear  that  the  transpiring  power  of  the  dorsal  surface 
is  low,  perhaps  indicating  cuticular  transpiration  only,  but  that  of  the 
ventral  surface  is  high.  Similar  results  were  obtained  on  August  22, 
in  which  a  difference  of  about  8  to  1  in  the  index  was  found  between 
the  ventral  and  the  dorsal  surfaces  of  the  same  leaf. 

On  August  23  tests  were  made  on  the  ventral  surface  only.  They 
were  carried  out  in  part  on  numbered  leaves,  so  that  the  behavior 


126 


FEATURES  OF  THE  VEGETATION  OF  THE 


of  the  individual  leaf  through  the  day  was  learned,  and  this  was 
checked  by  tests  of  a  number  of  leaves  not  used  otherwise.  Of  the 
numbered  leaves,  No.  2  was  “young”  and  No.  3  was  “old,”  at  least, 
the  former  was  smaller  than  the  latter  and  may  have  been  developed 
the  year  the  study  was  carried  on.  The  first  tests  of  the  day  were  made 
about  sunrise  and  the  last  were  made  nearly  an  hour  after  the  sun  had 
set  on  the  side  of  the  kopje  where  the  plant  studied  was  growing. 
The  day  was  clear  throughout,  and  the  temperature  of  the  air  varied 
from  8°  to  23°  C.  A  summary  of  the  results,  including  the  time  of 
observations  and  the  indices  of  the  transpiration  power  of  the  ventral 
surface,  is  given  in  table  17. 


Table  17. — Transpiring  power  of  Grewia  cana,  ventral  leaf -surf ace,  August  23. 


Large 

leaf. 

Small 

leaf. 

Leaves  all 
different. 

Large 

leaf. 

Small 

leaf. 

Leaves  all 
different. 

7h  36m . 

0.132 

4h  50m . 

0.138 

0.092 

9  20  . 

0.201 

5  5  . 

0.116 

10  12  . 

.160 

.152 

.116 

10  22  . 

.114 

.117 

11  18  . 

.167 

.119 

.134 

2  25  . 

.237 

.207 

.145 

2  45  . 

0.260 

5  47  . 

.090 

.111 

.216 

5  55  . 

.141 

.173 

.147 

.173 

The  first  observation,  as  indicated  in  the  table,  was  on  a  large  leaf, 
when  an  average  index  of  transpiring  power  of  0.132  was  obtained. 
At  the  same  time  another  test  was  run  on  a  large  leaf,  which  was 
partly  protected  by  a  Gymnosporia  shrub  which  was  close  by.  This 
had  a  reaction  time  of  480  seconds,  the  longest  of  the  day,  and  the 
index  was  0.097. 

It  was  frequently  observed  that  successive  tests  of  the  transpiring 
power  of  the  ventral  surface  were  unlike,  although  the  external  con¬ 
ditions  were  apparently  unchanged.  Thus  at  7h  36m  the  successive 
reaction-times  were  345  and  360  seconds  and  at  8h  5m  successive  reac¬ 
tion-times  of  240  and  270  seconds  were  obtained  on  the  same  leaf. 
At  9h  40m  two  tests  were  made  4  minutes  apart  on  the  same  leaves 
and  on  the  same  leaf  area.  In  the  case  of  a  smaller  leaf  the  reaction- 
times  were  90  and  150  seconds,  and  in  a  larger  leaf  they  were  100  and 
180  seconds.  The  differences  in  the  reaction-time  are  consequently 
in  the  direction  above  noted.  Such  results  indicate  that  in  some  way 
the  presence  of  the  glass  clips  disturbs  the  series  of  events  which  accom¬ 
pany  water  movements  within  the  plant  and  through  which  the 
moisture  is  brought  into  contract  with  the  surrounding  air.  What 
these  may  be  was  not  studied.  But  it  is  clear  that  the  clips  cut  out 
temporarily  the  saturation  deficit  of  the  atmospheric  air  as  an  imme- 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


127 


diate  factor  influencing  water  movement  within  the  plant,  as  well  as 
to  greatly  change  the  character  of  the  light  falling  on  the  leaf-surface. 

Gymnosporia  buxifolia? 

Gymnosporia  buxifolia  (plate  7a,  7c) 1  is  an  evergreen  shrub  which 
occurs  at  Beaufort  West,  on  the  southern  slope  of  kopjes  near  town. 
The  leaves  are  3  or  4  cm.  long  and  about  1  cm.  in  width,  and  the 
two  leaf-surfaces  are  unlike  in  appearance.  The  specimens  studied 
were  growing  in  association  with  Grewia  cana,  of  which  the  transpira¬ 
tion  power  was  treated  in  the  preceding  section,  and  to  which  it  bears 
resemblance  in  its  general  habit  of  growth. 

The  transpiring  power  of  Gymnosporia  was  studied  on  August  5, 
6,  12,  13,  and  17;  89  observations  wTere  made,  special  attention  being 
paid  to  possible  differences  in  the  reaction  of  the  two  leaf-surfaces. 
All  of  the  tests  were  made  during  the  hours  of  sunlight,  although  those 
on  August  13  were  begun  at  about  sunrise.  The  days  were  without 
cloud.  On  the  17th  there  was  a  strong  breeze  and  the  air  was  rela¬ 
tively  very  dry.  It  requires  480  seconds  for  the  dry  and  dark-blue 
cobalt-chloride  paper  to  fade  to  light  blue  when  freely  exposed  to  the 
air. 

Preliminary  tests  did  not  show  consistent  differences  in  the  trans¬ 
piring  power  of  the  leaf-surfaces,  although  the  dorsal  appeared  to  have 
the  larger  index.  Table  18,  in  which  the  time  of  observation  and  the 
indices  of  transpiring  power  are  given,  summarizes  the  results  of 
August  13. 


Table  18. — Transpiring  power  of  dorsal  and  ventral  leaf -surf aces, 
Gymnosporia  buxifolia,  August  13. 


Dorsal  leaf-surface. 

Ventral  leaf-surface. 

Leaf  No.  1. 

Leaf  No.  2. 

Leaf  No.  3. 

Leaf  No.  4. 

7  to  8  hours . .  . 

0.142 

0.137 

0.117 

0.095 

9  to  10  hours . .  . 

.133 

.146 

.128 

.078 

11  to  12  hours .  . . 

.240 

.240 

.202 

.061 

12  to  1  hours . .  . 

.100 

.100 

.151 

.098 

2  to  3  hours .  . . 

.072 

.108 

.103 

.056 

On  August  17  lower  values  were  obtained.  The  reaction- time 
varied  from  210  mid-forenoon  to  3,360  seconds  in  mid-afternoon,  and 
the  index  of  transpiring  power  ran  from  0.023  to  0.065.  These  do  not 
represent  possible  extremes  of  the  day,  because  readings  were  not 
made  in  early  morning,  when  a  high  index  might  be  expected.  At 
10h  25m  the  dorsal  surface  of  leaf  No.  3  had  an  index  of  0.065,  and  the 
ventral  surface  of  No.  4,  a  similar  leaf,  had  an  index  of  0.030  and  0.052. 

1  Dr.  Marloth  informs  me  that  this  may  be  G.  integrifolia  (L.  F.)  Szysz. 


128 


FEATURES  OF  THE  VEGETATION  OF  THE 


At  llh  55m  the  index  of  the  dorsal  surface  of  leaf  No.  2  was  0.023,  and 
at  12h  7m  that  of  the  ventral  surface  of  the  same  leaf  was  0.041.  These 
differences  in  the  size  of  the  index  of  transpiring  power  as  between 
the  dorsal  and  the  ventral  surfaces  are  not  sufficiently  large  or  consist¬ 
ent  to  make  it  appear  likely  that  the  leaves  are  physiologically  dorsi- 
ventral,  although  they  have  this  appearance.  The  structure  will  be 
commented  on  in  another  place. 

Massonia  latifolia. 

At  Beaufort  West  the  acaulescent  Massonia  latifolia  (plate  9)  with 
water  storage  in  the  subterranean  organs  occurs  on  the  southern  slopes 
of  a  kopje  about  2  miles  north  of  town.  The  opposite  leaves  lie 
freely  on  the  surface  of  the  ground  and  are  about  10  by  20  cm.  in  size 
and  possibly  4  mm.  in  thickness. 

The  transpiration  power  of  the  plant  was  observed  on  August  19 
and  22,  when  numerous  tests  were  made.  On  the  first  day  attention 
was  given  to  determination  of  possible  differences  in  the  two  surfaces 
in  transpiration  power.  On  the  second  day  only  the  upper  leaf- 
surface  was  used.  The  time  of  observation  on  August  19  and  the 
calculated  indices  are  summarized  in  table  19. 


Table  19. — Transpiring  power  of  Massonia  latifolia,  August  19. 


Index  of  upper 
leaf-surface. 

Index  of  lower 
leaf -surface. 

Index  of  upper 
leaf-surface. 

Index  of  lower 
leaf-surface. 

9h  57m . 

0.171 

3h  58m . 

0.088 

.097 

10  20  . 

0.125 

.154 

.257 

4  10  . 

10  42  . 

.052 

0.088 

.081 

.065 

4  22  . 

11  8  . 

.033 

4  34  . 

.052 

On  August  22  the  upper  surface  of  4  leaves  was  used.  The  first 
observations  were  made  about  35  minutes  before  the  sun  rose  on  the 
south  side  of  the  kopje  and  were  continued  until  between  12  and  1 
o'clock.  The  day  was  clear  and  the  temperature  of  the  air  varied 
from  7°  in  early  morning  to  20°  C.  at  midday.  Reaction-times 
between  150  and  2,500  seconds  were  obtained,  with  corresponding 
differences  in  the  index  of  transpiring  power.  Table  20,  giving  the 
time  of  observations  and  the  indices,  summarizes  the  results. 

Table  20. — Transpiring  power,  dorsal  leaf -surf ace,  of  Massonia  latifolia,  August  22. 


Leaf  No.  1. 

Leaf  No.  2. 

Leaf  No.  3. 

Leaf  No.  4. 

7h  26m . 

0 . 0206 

0.018 

9h  to  10h  . 

0.048 

0.046 

.047 

llh  to  llh  37m .  .  . 

.161 

.114 

.100 

.0503 

.050 

.047 

12h  15m . 

.066 

.066 

.083 

MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


129 


It  appears,  therefore,  that  the  index  of  transpiring  power  is  less 
in  early  morning  than  in  late  forenoon,  and  that  during  so  short  time 
its  variation  may  be  considerable. 

There  was  no  apparent  difference  in  transpiring  power  between  the 
lower  and  protected  surface  and  the  upper  exposed  surface. 

Matjesfontein. 

The  species  used  at  Matjesfontein  as  a  center  were  situated  in  part 
on  veld  and  in  part  in  an  abandoned  park  in  town.  In  the  latter, 
water  from  the  rains  only  was  received.  Aloe  striata ,  Cotyledon 
coruscans,  C.  paniculata,  Eucalyptus  globulus ?,  and  an  undetermined 
weed  belonging  to  the  Chenepodiacese  were  studied  in  the  location  last 
named.  Cotyledon  paniculata  was  also  studied  on  the  veld,  as  were 
Euclea  undulata ,  Euyrops  lateriflorus ,  and  Rhus  viminalis.  A  species 
of  Rhus  was  also  studied  atWhitehill,  and Proteaneriifolia at  Tweedside. 

Aloe  striata. 

The  leaf  succulent  Aloe  striata  is  a  large-leaved  species  occurring 
further  east  in  the  Great  Karroo,  which  has  been  introduced,  together 

Table  21. — Transpiring  power  of  Aloe  striata ,  August  24. 


Younger  leaf. 

Older  leaf. 

6h  6m  to  18m . 

0.051 

0.060 

Sh  16m  to  20m . 

.051 

.040 

10h  16m  to  20m . 

.016 
°  .019 
°  .018 

.028 

10h  2m  to  32m . 

.011 

.012 

.010 

12h  to  12h  15m  .... 

.012 

.012 

lh  50m  . 

b  .025 

b  .008 

4h  2m  to  15m . 

.013 

.010 

a  Two  leaves  not  used  at  any  other  time. 
b  The  index  for  the  younger  leaf  is  somewhat  too  high,  and 
that  of  the  older  leaf  somewhat  too  low. 

with  numerous  other  Karroo  species,  in  the  park  at  Matjesfontein. 
Here  it  grows  well,  although  the  rainfall  is  less  than  in  the  Gouph,1 
and,  moreover,  occurs  mainly  at  another  season. 

Several  tests  were  made  on  Aloe  striata ,  but  only  those  of  October 
24  will  be  described.  Sunrise  occurred  on  that  day  about  the  time  of 
the  first  observations.  The  morning  was  clear,  but  about  noon  light 
clouds  appeared  and  the  sky  became  hazy.  The  temperature  of  the 
air  varied  between  8°  C.  in  early  morning  and  34°  C.  in  mid-afternoon. 
Two  leaves  were  mainly  used.  Of  these,  one  was  smaller  and  prob¬ 
ably  younger  than  the  other.  The  time  of  the  observations  and  the 
index  of  the  transpiring  power  are  given  in  summarized  form  in  table  21. 

1  The  central  portion  of  the  Great  Karroo.  A  Hottentot  word  said  to  signify  “empty,  bald, 
naked,  or  nothing,”  according  to  Bews. 


130 


FEATURES  OF  THE  VEGETATION  OF  THE 


Cotyledon  cortjscans. 

Cotyledon  coruscans  (plate  24c)  is  a  leaf  succulent  of  rather  wide 
distribution  in  the  Karroo.  The  evergreen  leaves  are  fairly  thick  and 
about  7  by  14  cm.  in  size.  The  species  is  native  at  Matjesfontein, 
where  it  occurs  in  stony  places,  on  kopjes,  etc. 


Fig.  11. — Average  indices  of  foliar  transpiring  power  at  2-hour 
intervals,  8h  a.  m.  to  10h  p.  m.  A,  Aloe  schlechteri;  B, 
Gasteria  disticha;  C,  Aloe  striata;  D,  Cotyledon  canescens. 


The  transpiration  power  of  the  species  was  studied  October  5,  6, 
and  13.  Tests  were  made  on  specimens  growing  on  the  edge  of  the 
town.  Controls  were  used.  On  October  5  and  6  the  weather  was  fair 
and  the  air  was  dry,  so  that  the  control  slips  changed  little  or  not  at  all. 
On  October  13,  however,  there  were  some  clouds  and  the  controls 
underwent  reaction  to  light  blue  and  to  pink  within  30  minutes. 
Table  22,  in  which  the  observation  time  and  the  index  of  transpiration 
power  are  given,  summarizes  the  leading  results. 

It  will  be  noticed  that  there  was  a  certain  degree  of  agreement  in  the 
hourly  course  of  the  index  of  transpiring  power,  during  the  hours  of 


Table  22. — Transpiring  power  of  Cotyledon  coruscans. 


Oct.  5. 

Oct.  6. 

Oct.  23. 

7  to  8  hours .... 

0.0140 

0.0120 

8  to  9  hours .... 

0 . 0099 

9  to  10  hours . . . 
10  to  11  hours . .  . 

.0100 

.0073 

.0100 

.0096 

.0070 

1 1  to  12  hours . .  . 

.0097 

12  to  1  hours. .  .  . 

.0110 

2  to  3  hours .... 

.0033 

.0053 

4  to  5  hours .... 

.0062 

.0110 

.0080 

5  to  6  hours .... 

.0090 

6  to  7  hours. .  .  . 

.0160 

.016 

7  to  8  hours .... 

.016 

9  to  10  hours. .  . 

.0160 

MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


131 


daylight  at  least,  in  the  three  days.  At  or  before  sunrise,  and  about 
or  after  sunset,  the  index  is  relatively  high,  but  it  falls  during  the  early 
portion  of  the  afternoon. 

There  is  also  a  striking  correspondence,  but  not  exact  agreement, 
between  the  reaction  of  the  control  and  that  of  the  test  clips.  When 
the  indices  were  low  the  reaction-time  of  the  control  was  long.  But 
in  certain  tests,  as  at  9h  50m  on  October  13,  the  reaction-time  of  the 
test  clip,  from  dark  to  light  blue,  was  2,820  seconds,  while  the  reaction 
time  of  the  control,  dark  blue  to  pink,  was  6,000  seconds.  The  control 
was  exposed  to  the  wind,  but  the  plant  was  sheltered.  It  is  evident, 
therefore,  that  while  the  transpiring  power  of  the  plant  was  not  large, 
some  transpiration  in  fact  did  occur. 

Cotyledon  paniculata. 

Cotyledon  paniculata  (plate  18)  occurs  sparingly  on  kopjes  in 
the  vicinity  of  Matjesfontein.  This  species  has  features  of  much 
interest.  As  is  shown  elsewhere  in  this  study,  it  is  a  stem  succulent 


Fig.  12. — Average  indices  of 
foliar  transpiring  power  at 
2-hour  intervals,  8h  a.  m.  to 
10h  p.  m.  A,  Cotyledon  pan- 
icularial ;  B,  Massonia  lati- 
folia. 


having  fleshy  stem  and  branches,  and  fairly  fleshy  leaves  which  are 
deciduous.  Leaves  are  present  in  winter  and  early  spring,  but  they 
fall  with  the  approach  of  the  warm  season  and  the  species  passes  the 
summer  in  a  leafless  condition.  In  September,  when  studies  were 
begun  on  the  species,  the  leaves  appeared  to  be  turgid  and  photosyn- 
thetically  active,  but  in  middle  of  October,  and  especially  later  in  the 
month,  those  on  some  specimens  were  beginning  to  give  evidence  of 
preparation  for  early  falling. 

The  specimens  used  in  the  studies  were  mainly  in  the  abandoned 
park  in  town,  but  some  observations  were  also  carried  out,  in  October 
only,  on  plants  growing  on  a  kopje  about  2  miles  distant.  Studies 
on  the  transpiration  of  C.  paniculata  were  made  on  September  21, 
26,  and  27,  and  October  4,  13,  and  19.  A  running  account  will  be 
given  of  those  which  are  considered  most  important,  a  resume  of  the 
balance,  and  a  summary  of  all. 

The  plant  used  September  21  was  growing  on  a  small  pile  of  rather 
small  rocks,  where  it  had  been  brought  from  a  nearby  kopje.  The 
main  stem  was  about  8  cm.  in  diameter  and  bore  fleshy  branches. 
The  leaves  studied  were  either  about  8  by  5  cm.  or  about  4.5  by  2.2  cm. 
in  size,  the  smaller  being  in  general  nearer  the  top  of  the  plant.  The 
large  leaves  were  about  5  mm.  in  thickness  and  were  not  very  turgid; 
at  least  they  were  not  easily  broken  when  bent.  Young  shoots  bearing 


132 


FEATURES  OF  THE  VEGETATION  OF  THE 


spatulate  and  small  leaves  were  appearing.  Only  the  leaves  of  the 
preceding  season,  however,  were  used  in  the  studies. 

On  September  21  tests  were  made  between  6h  35m  in  the  morning 
and  4h  in  the  afternoon.  Sunrise  occurred  at  about  6h  45m.  There 
was  sunshine  until  noon,  when  light  clouds  appeared,  and  much  of 
the  afternoon  was  somewhat  overcast. 

The  transpiration  power  of  the  dorsal  surface  only  of  the  leaves  was 
studied. 

Four  tests  were  made  about  sunrise,  with  reaction-time  of  540, 
700,  720,  and  720  seconds.  The  average  index  of  transpiring  power 
was  0.0648.  At  7h  44“  and  8h  2“  the  reaction-time  was  found  to  vary 
from  720  to  1,020  seconds,  and  the  average  index  was  0.033. 

From  9h  29“  three  tests  were  made,  giving  reaction-time  of  660, 
1,020,  and  1,140  seconds.  The  average  reaction-time  of  the  last 
two  is  0.0163,  that  of  the  first  is  0.023.  Of  the  three  observations, 
the  longest  is  noted  as  being  “good/’  the  intermediate  one  as  possibly 
having  been  continued  overlong,  but  the  first  was  “satisfactory.” 

At  10h  18“  to  10h  24“  the  reaction- time  for  three  tests  was  from 
780  to  1,260  seconds  and  the  average  index  was  0.0149.  At  10h  24“ 
a  single  test  (probably  of  an  old  leaf)  had  a  reaction-time  of  2,160 
seconds,  at  a  temperature  of  24°  C.  The  index  of  the  transpiring 
power  for  this  test  was,  therefore,  0.0072. 

Between  llh  44“  and  12h  7“  three  tests  gave  reaction-time  of  600, 
600,  and,  on  an  old  leaf,  1,280  seconds.  The  index  of  the  former  is 
0.028  and  of  the  latter  0.0098. 

Three  tests,  beginning  with  2 h  14“,  gave  reaction-time  of  1,800  to 
2,100  and  an  average  index  of  0.0063. 

And,  finally,  three  observations  beginning  at  4h,  when  there  were 
clouds,  gave  reaction- time  between  1,320  and  1,740  seconds  and  an 
average  index  of  the  transpiring  power  of  0.0147. 

During  the  day  observations  were  also  made  on  the  transpiring 
power  of  a  species  of  chenopod  which  was  growing  within  1  meter 
of  the  species  of  Cotyledon  studied.  An  index  between  0.075  and 
0.19  was  obtained  for  this  plant. 

On  September  26  observations  were  made  from  6h  45“  in  the  morn¬ 
ing,  that  is,  from  about  sunrise,  until  5h  3“  in  the  evening.  The  day 
was  with  sunshine  throughout  and  very  dry.  Control  slips  were 
used  paralleling  each  test  period.  They  did  not  change  in  shade  of 
color  at  any  time  during  the  day,  except  only  that  at  about  midday 
they  appeared  to  get  somewhat  lighter  than  when  they  were  put  out. 

At  6h  45“  five  tests  were  made.  The  same  plants  were  used  as  on 
the  preceding  day.  After  60  minutes  there  was  no  color  change. 
At  9h  5“  tests  were  again  made.  The  cobalt-chloride  papers  at  the 
end  of  50  minutes  were  pink  on  the  edges,  but  light-blue  on  the  inside. 
The  control  was  unchanged. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


133 


Five  tests  were  again  made  at  llh  5m,  when  a  reaction-time  of  1,800 
and  2,400  seconds  was  noted,  and  at  1  lh  16m  a  third  test  gave  a  reaction¬ 
time  of  2,880  seconds.  In  the  first  instance  the  index  was  0.0011, 
and  in  the  last  test  it  was  0.0086. 

At  2h  15m  a  reaction-time,  the  average  of  five  tests,  of  2,280  seconds 
was  obtained,  giving  0.0077  as  the  index  of  transpiring  power. 

Two  observations,  of  two  tests  each,  were  made  at  4h  13m,  when  the 
average-reaction  time  was  1,620  seconds,  and  the  index  was  0.0109. 
This  was  on  a  relatively  young  leaf.  At  the  same  time  a  series  was 
run  on  an  older  leaf,  in  which  the  reaction-time  was  2,940  seconds, 
giving  the  index  as  0.0059. 

At  5h  3m  the  index  was  determined  to  be  0.006. 

Observation  about  5  o’clock  on  the  transpiration  of  a  chenopod 
growing  near  showed  reactions  between  180  and  420  seconds,  with 
indices  between  0.11  and  0.041. 

The  tests  made  on  September  27,  which  was  also  a  clear,  dry  day, 
gave  essentially  the  same  results  as  on  the  preceding  days,  although 
the  first  observation  was  not  made  until  8h  25m. 

The  index  of  the  transpiring  power  for  the  earlier  tests  was  0.011, 
which  was  the  average  of  five  observations.  At  about  10  o’clock  the 
index  had  fallen  to  0.0055,  but  this  is  probably  too  low,  as  the  reac¬ 
tion  was  somewhat  beyond  the  end  customarily  used.  At  about 
noon  the  average  index  of  five  tests  was  0.01.  At  2h  9m  the  average 
of  five  tests  gave  an  index  of  0.009.  At  4h  7m  five  tests  were  run,  of 
which  two  gave  an  index  of  0.0048  and  one  0.0087.  At  6h  7m  the  final 
observations  were  made,  at  which  time  the  average  index  for  five 
observations  was  0.012. 

At  the  time  the  observations  were  being  made  on  the  cotyledon, 
tests  were  run  on  leaves  of  Eucalyptus  globulus  ?  and  on  a  species  of 
Chenepodium,  both  of  which  were  within  about  150  cm.  of  the  coty¬ 
ledon.  The  index  of  transpiring  power  of  the  chenopod  at  10  o’clock 
was  0.265,  and  that  of  an  old  and  round  leaf  of  the  Eucalyptus  was 
0.168,  while  that  of  another  Eucalyptus  leaf,  which  was  younger,  was 
0.0808.  At  about  4  o’clock  a  round  and  old  leaf  of  Eucalyptus ,  the 
same  as  above  used,  gave  an  index  of  0.082,  and  the  younger  round 
leaf  an  index  of  0.061.  The  older  leaf,  at  6h  12m  had  an  index  of  0.0248. 

On  October  3  observations  were  begun  at  6h  45m,  but  a  heavy  wind 
sprang  up  with  the  rising  of  the  sun  and  the  humidity  of  the  air 
rapidly  increased,  so  that  the  control  slips  changed  in  color  fairly 
quickly,  as  did  also  those  on  the  plants  studied.  The  tests  were, 
therefore,  abandoned  for  the  time. 

The  index  of  the  transpiring  power  of  the  Cotyledon,  as  determined 
on  October  4,  was  for  each  period  given  as  follows:  7h  50m,  0.013; 
9h  44m,  0.019;  10h  34m,  0.011;  12h  15m,  0.011;  2h  8m,  0.0089;  4h  17m, 
0.01 ;  and  5h  43m,  0.02.  Control  clips  were  used  during  the  day  and 
they  showed  no  or  slight  change  in  color. 


134 


FEATURES  OF  THE  VEGETATION  OF  THE 


On  October  13  the  weather  conditions  were  somewhat  variable. 
The  morning  was  clear,  but  toward  mid-day  light  clouds  appeared 
and  the  air  was  as  usual  dry,  although  there  was  some  lack  of  uni- 
formity  in  this  regard.  For  example,  in  early  morning  the  controls 
changed  in  color  little,  if  any,  but  at  llh  25m  a  control  paper  underwent 
complete  reaction,  that  is,  to  pink,  within  a  few  minutes.  In  the 
afternoon,  however,  the  controls  changed  only  slightly. 

Leaves  Nos.  1,  2,  6,  8,  and  9  were  studied.  Before  and  about 
sunrise  five  tests  were  made  of  which  the  earliest,  6h  5m  gave  the  highest 
index,  namely,  0.05,  which  was  of  Nos.  6  and  8.  A  half  hour  later 
No.  2  had  an  index  of  0.045.  But  the  balance  had  indices  between 
0.023  and  0.029. 

At  8h  8m  the  index  of  Nos.  1  and  2  was  0.023,  and  of  Nos.  6  and  8 
was  0.0152.  The  control  which  was  running  parallel  to  these  tests 
did  not  show  any  change  in  color. 

The  average  index  for  three  tests  made  at  9h  35m  was  0.017.  At  this 
time  the  control  had  changed  but  slightly. 

At  llh  20m  three  observations  were  made.  Of  these  No.  9  had  an 
index  of  0.0091,  and  Nos.  2  and  6,  0.0073.  The  control  during  this 
period  was  completely  changed  in  color,  that  is,  to  pink. 

At  2h  12m  the  average  index  of  Nos.  1,  2,  and  6  was  0.014.  The 
control  had  changed  slightly  in  color. 

The  final  test  was  made  at  4h  8m  with  Nos.  2,  6,  and  9,  when  the 
average  index  was  found  to  be  0.014.  The  control  had  partly  changed 
during  the  period. 

On  October  19  a  few  tests  were  run  on  two  plants  of  Cotyledon 
paniculata  growing  on  a  kopje  about  2  miles  northeast  of  Matjes- 
fontein.  They  were  about  1  meter  high.  The  diameter  of  the  stem 
of  one  was  about  16  cm.  and  that  of  the  other  about  39  cm.  The 
leaves  of  the  tips  of  some  of  the  branches  were  somewhat  yellow,  but 
those  used  were  green  and  apparently  quite  vigorous. 

Observations  were  made  2h  35m,  3h  5m,  and  4h  35m.  The  controls 
did  not  change  in  color  during  the  tests. 

At  2h  35m  six  leaves  were  used.  They  had  an  average  reaction-time 
of  1,500  seconds  and  an  average  index  of  transpiring  power  of  0.0093. 
The  average  reaction-time  of  the  next  following  observation  was  2,280 
seconds  and  the  index  was  0.0072.  At  4h  35m  the  average  reaction¬ 
time  of  three  leaves  was  0.011.  The  reaction  had  been  carried  some¬ 
what  past  the  light-blue  color  of  two  other  leaves.  It  is  evident  that 
the  index  of  transpiring  power  of  the  species  growing  on  the  kopje 
was  not  far  from  that  of  the  species  in  the  neglected  park  at  Matjes- 
fontein,  so  that  the  results  with  the  latter  plants  can  be  interpreted 
as  also  applying  to  species  in  the  veld. 

It  will  be  seen  from  the  summary  of  the  indices  of  transpiring 
power  given  in  table  23  that  the  average  index  for  Cotyledon  paniculata 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


135 


decreases  from  early  morning,  and  fairly  quickly  so,  so  that  by  mid¬ 
forenoon  it  may  be  at  the  lowest  for  the  day.  It  increases  again  toward 
evening. 

As  in  certain  other  species,  particularly  with  C.  coruscans,  there 
was  sometimes  noted  a  parallel  in  behavior  as  between  the  control 
and  the  hygroscopic  paper  used  in  the  tests.  For  example,  in  early 
morning  on  September  26  the  reaction-time  was  more  than  3,600 
seconds,  and  there  was  no  color  change  in  the  control.  But,  on  the 
other  hand,  on  the  following  day,  at  llh  55m,  the  reaction-time  was 
1,550  seconds,  and  only  slight  change  was  seen  to  have  taken  place 
in  the  control. 

Tests  of  the  transpiring  power  of  Cotyledon  paniculata,  Eucalyptus 
globulus  ?,  and  of  a  species  of  chenopod,  which  were  run  synchronously 
on  September  27,  gave  interesting  comparative  results.  Thus  the 
reaction-time  of  Cotyledon  was  3,600  seconds,  that  of  the  chenopod 
was  180  seconds,  and  that  of  old  and  round  leaves  of  the  Eucalyptus 
was  120  and  of  younger  round  leaves  240  seconds.  The  indices  of 
the  transpiring  power  of  these  tests  were  0.0055,  0.117,  0.176,  and 
0.088,  respectively.  This  series  suggests,  which  is  further  strength¬ 
ened  by  the  table  of  indices  and  that  given  in  connection  with  the 


Table  23. — Transpiring  power  of  ( indices )  Cotyledon  paniculata. 


Sept.  21. 

Sept.  26. 

Sept.  27. 

Oct.  4. 

Oct.  13. 

Oct.  19. 

6  to  7  hours . 

0.064 

0.0006 

0.011 

7  to  8  hours . 

.035 

0.019 

8  to  9  hours . 

0.011 

.019 

9  to  10  hours . 

.017 

.009 

.005 

.021 

.008 

10  to  11  hours . 

.011 

.008 

11  to  12  hours . 

.017 

.008 

.009 

.008 

12  to  1  hours . 

.010 

2  to  3  hours . 

.005 

.007 

.009 

.008 

.015 

0.009 

3  to  4  hours . 

.007 

4  to  5  hours . 

.014 

.005 

.006 

.009 

.014 

.011 

5  to  6  hours . 

.016 

6  to  7  hours . 

.012 

• 

paragraphs  on  Eucalyptus ,  that  the  highest  index  of  transpiring  power 
of  the  Cotyledon  is  usually  lower  than  the  lowest  of  the  Eucalyptus, 
and  that  the  difference  at  the  same  time  may  in  fact  be  very  consid¬ 
erable  indeed. 


Eucalyptus  globulus? 

As  has  already  been  mentioned,  Eucalyptus  globulus ?  occurs  in  an 
abandoned  park  at  Matjesfontein.  It  was  not  irrigated  and  appar¬ 
ently  had  not  been  irrigated  for  several  years  previous  to  my  visit. 
The  trees  are  15  meters,  more  or  less,  in  height  and  at  the  base  are 
young  shoots  bearing  the  juvenile  type  of  leaves.  Both  the  narrow  and 


136 


FEATURES  OF  THE  VEGETATION  OF  THE 


the  round  forms  of  leaves  were  studied  as  an  aside  while  the  transpiring 
power  of  Cotyledon  coruscans  and  C.  paniculata  was  being  investigated. 

Observations  on  the  transpiring  power  of  Eucalyptus  were  made  on 
September  27  and  October  4  and  13.  The  results  of  the  tests  of 
September  27  have  already  been  referred  to.  It  was  stated,  in  brief, 
that  the  index  of  the  transpiring  power  of  round  and  old  leaves  at 
about  10  o’clock  in  the  morning  was  0.168,  and  that  of  a  younger  leaf 
was  at  the  same  time  0.080.  In  the  afternoon  at  about  4  o’clock  the 
index  of  the  old  leaf  was  0.082  and  of  the  younger  leaf  was  0.061. 
At  about  6  o’clock  the  index  of  the  older  leaf  was  0.Q248. 

On  October  4  the  observations  on  Eucalyptus  were  made  between 
about  noon  and  before  6  o’clock  in  late  afternoon.  Four  leaves  were 
used.  Of  these  Nos.  1  and  2  were  round  but  mature  leaves  growing 
on  a  young  shoot.  They  were  about  3.5  by  4.5  cm.  in  size.  Leaves 
3  and  4  were  of  narrow  type  and  were  on  an  old  branch.  No.  3  meas¬ 
ured  4.4  by  8.5  cm.  and  No.  4  was  9  by  4.2  cm. 

At  llh  50m  the  average  reaction-time  of  several  readings  of  leaves 
1  and  2  was  60  seconds,  while  that  of  No.  3  was  185  seconds,  giving  the 
index  of  transpiring  power  of  0.32  in  the  former  instance  and  of 
0.098  in  the  latter. 

Table  24. — Transpiring  power  of  Eucalyptus  globulus ?,  October  13. 


Leaf  No.  1. 

Leaf  No.  2. 

6h  37m . 

0.082 

0.082 

8  15  . 

.085 

.050 

9  40  . 

.072 

.050 

11  27  . 

.097 

.075 

2  18  . 

.184 

.070 

4  12  . 

.095 

.025 

Leaves  Nos.  1  and  2  at  2h  10m  had  a  reaction- time  of  from  80  to  90 
seconds.  The  index  of  transpiring  power  was  0.19.  At  2h  25m 
leaves  Nos.  3  and  4  had  a  reaction- time  of  150  seconds  and  an  index 
of  0.104.  At  4h  20m  the  index  of  reaction-time  of  Nos.  1  and  2  was 
0.0193.  At  this  time  the  reaction- time  of  leaf  No.  4  was  100  and  of 
No.  3  was  300  seconds,  giving  the  indices  of  0.16  and  0.054.  The 
final  reading  was  at  5h  45m,  at  which  time  leaves  Nos.  1  and  2  had 
a  reaction-time  of  59  and  49  seconds  and  an  average  index  of  0.36.  The 
reaction-time  of  leaf  No.  4  at  this  time  was  120  seconds  and  the  index 
was  0.13. 

On  October  13  leaves  Nos.  1  and  3  were  used.  Observations  were 
made  between  6h  37m  in  the  morning  and  4h  12m  in  the  afternoon.  The 
character  of  the  day  has  been  characterized  in  connection  with  an 
account  of  the  transpiring  power  of  Cotyledon  paniculata.  Table  24 
gives  the  indices  with  the  hour  of  each  observation. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


137 


It  will  be  seen,  therefore,  that  except  for  the  early  morning  reading, 
the  index  of  transpiring  power  of  the  mature  (elongated)  form  of  leaf 
is  the  smaller  at  each  observation.  Comparing  the  indices  of  Euca¬ 
lyptus  with  those  for  Cotyledon  paniculata,  which  were  for  the  same  day, 
it  will  be  found  that  those  of  the  latter  are  consistently  below  the 
indices  of  the  mature  form  of  Eucalyptus  leaf. 

Euryops  lateriflorus. 

The  shrub  Euryops  lateriflorus  (plate  15)  occurs  sparingly  on  a 
rocky  outcrop  on  the  outskirts  of  Matjesfontein.  The  leaves  are 
coriaceous,  about  6  by  15  mm.  in  size,  and  are  closely  appressed  to 
the  branches.  They  are  abundant.  The  transpiring  power  was 
studied  on  several  days,  but  the  results  of  observations  made  on  Octo¬ 
ber  14  and  15  need  only  be  given. 

The  placing  of  the  leaves  by  which  the  dorsal  surface  is  pressed 
closely  against  the  upright  branches,  with  the  effect  that  the  ventral 
surface  only  is  fully  exposed  to  the  sunlight  and  the  air-currents  led 
to  the  supposition  that  the  two  leaf-surfaces  might  have  unlike  trans¬ 
piring  power.  Accordingly  observations  were  made  on  both  surfaces. 

On  October  14  the  day  was  fair  and  the  controls  showed  that  the 
air  was  fairly  dry.  Wind  came  from  the  westerly  direction.  Sunrise 
was  about  6h  10m,  about  an  hour  after  which  the  observations  were 
begun.  They  were  continued  until  6  o’clock  in  the  evening.  The 
leading  results  are  given  in  table  25. 

Table  25. — Transpiring  power  of  Euryops  lateriflorus,  October  14. 


Dorsal  leaf- 
surface. 

Ventral  leaf- 
surface. 

7h  34m . 

0.028 

0.057 

9  23  . 

.029 

.032 

9  47  . 

.033 

.028 

11  34  . 

.045 

.030 

1  53  . 

.030 

.030 

2  22  . 

.023 

.023 

4  31  . 

.032 

.029 

6  . 

.018 

.015 

On  the  following  day,  in  order  to  further  test  the  transpiring  power 
of  the  two  leaf-surfaces,  observations  were  conducted  on  both  sur¬ 
faces  of  different  leaves  and  of  the  same  leaf.  Tests  also  were  made  of 
leaves  which  had  been  in  the  direct  sunlight  for  a  few  minutes  as 
opposed  to  such  as  had  not  as  yet  been  in  the  direct  sunlight.  The 
day  was  about  as  the  preceding  one  in  that  the  sun  was  not  obscured 
by  clouds  and  it  was  fairly  dry.  The  shrub  was  in  full  sunshine  at 
6h  22m. 

At  5h  57m  the  clips  were  placed  on  the  dorsal  and  on  the  ventral 
sides  of  two  separate  leaves  which  were  on  the  side  of  the  shrub  away 


138 


FEATURES  OF  THE  VEGETATION  OF  THE 


from  the  light.  The  index  of  the  dorsal  surface  was  determined  to  be 
0.032  and  of  the  ventral  surface  0.025. 

At  6h  30m  the  clips  were  placed  on  the  two  surfaces  of  different 
terminal  leaves,  which  had  been  about  20  minutes  in  direct  sunshine. 
The  dorsal  surface  side  gave  an  index  of  0.192  and  the  ventral  surface 
an  index  of  0.129. 

At  6h  42m  a  long  strip  of  the  hygrometric  paper  was  bent  and 
attached  to  the  leaf,  so  that  one  portion  was  on  the  dorsal  surtace 
and  the  other  portion  was  on  the  ventral  surface.  This  was  held 
in  position  by  a  glass  clip  with  color  standard  as  in  the  usual  way. 
At  the  expiration  of  300  seconds  the  reaction  of  the  portion  on  the 
dorsal  side  was  complete  light  blue,  while  the  reaction  of  the  portion 
of  the  paper  on  the  ventral  side  was  not  wholly  carried  through.  The 
experiment  was  repeated  at  8h  8m  and  later  with  similar  results. 

At  7h  55™  separate  leaves  on  the  shaded  side  of  the  shrub  were 
tested,  with  the  following  results:  The  index  of  transpiring  power  of 
the  dorsal  surface  was  0.167.  At  8h  56™  the  index  of  the  ventral 
surface  was  0.100. 

At  9h  15™  three  tests  were  made,  of  which  two  were  on  separate 
leaves  in  the  shade  and  one  was  of  a  terminal  leaf  that  had  been  in 
the  sun.  In  the  latter  instance  one  strip  of  paper  was  bent  and 
clamped  on  both  sides  of  the  leaf,  as  earlier  in  the  morning.  Of  the 
leaves  in  the  shade,  the  dorsal  surface  had  an  index  of  0.052  and  that 
of  the  ventral  surface  was  somewhat  less.  But  of  the  single  terminal 
leaf,  which  had  been  in  the  direct  sunlight,  the  index  of  the  two  surfaces 
was  the  same,  that  is,  0.091. 

The  ventral  leaf-surface  of  Euryops  lateriflorus ,  therefore,  has  an 
index  of  transpiring  power  which  may  or  may  not  be  lower  than  that 
of  the  dorsal  surface.  There  appears  to  be  some  sort  of  regulatory 
control  of  the  transpiring  power  of  the  lower  surface  which  is  absent 
or  not  so  pronounced  on  the  dorsal  surface.  In  any  event  the  dif¬ 
ferences  in  transpiring  power  between  the  two  leaf-surfaces  is  not  so 
striking  as  in  certain  other  species,  as,  for  example,  Grewia  cana,  Rhus 
viminalis,  or  Protea  neriifolia. 

Euclea  undulata. 

Euclea  undulata  is  a  shrub  with  small  coriaceous  leaves.  At  Mat- 
jesfontein  it  occurs  on  kopjes.  Only  one  series  of  observation  were 
carried  out  on  Euclea,  but  they  gave  certain  results  of  interest  and  will 
be  mentioned. 

The  specimen  observed  was  growing  on  the  summit  of  a  kopje 
about  2  miles  from  town  and  not  far  from  the  Cotyledon  paniculata 
which  was  studied  the  same  afternoon,  i.  e.,  October  19.  The  day 
was  warm,  25°  C.  shade  temperature,  and  dry.  The  control  slips 
did  not  undergo  color  change  dutihg  the  tests. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


139 


At  4h  8m  slips  were  attached  to  the  dorsal  and  ventral  surface  of 
5  leaves.  The  cobalt-chloride  paper  on  the  ventral  surface  changed 
color  in  5  minutes,  but  that  on  the  dorsal  surface  had  changed 
slightly,  if  at  all,  at  the  end  of  24  minutes.  The  index  of  transpiring 
power  of  the  ventral  surface  was  0.046  and  that  of  the  dorsal  surface 
less  than  0.0097.  It  is  evident,  therefore,  that  the  specimen  studied 
had  a  relatively  high  rate  of  transpiring  power,  but  that  there  was 
practically  no  loss  of  moisture  from  the  dorsal  surface  of  the  leaves. 

Rhus  sp.  and  R.  yiminalis. 

Two  species  of  Rhus  were  studied.  Of  these  one  is  a  tree,  Rhus 
viminalis ,  which  grows  along  the  streamway  at  Matjesfontein,  and  the 
other  is  a  shrub  which  is  apparently  confined  to  kopjes.  This  last 
was  observed  at  the  Karroo  Botanical  Gardens,  Whitehill,  where  it 
is  undisturbed  and  under  quite  natural  conditions. 

Table  26. — Transpiring  power  of  Rhus  viminalis,  October  20. 


Dorsal  surface 
of  leaf. 

Ventral  surface 
of  leaf. 

6h  16m . 

0.0452 

0.1590 

llh . 

.0337 

.2885 

2h . 

.0220 

.1137 

4h  32m . 

.0096 

.1020 

Although  the  tests  on  R.  viminalis  were  carried  out  on  several 
days,  those  of  October  20  only  will  be  given.  The  pendent  leaves 
of  the  species  are  long  and  narrow,  about  10  cm  by  7  mm.fj|They  are 
evidently  physiologically  dorsi-ventral,  even  if  the  two  leaf-surfaces 
are  not  in  appearance  strikingly  unlike.  |jj 

On  October  20  the  air  was  somewhat  humid  in  the  morning,  as  was 
indicated  by  the  reaction  of  the  control  papers  within  a  period  of  900 
seconds,  but  in  the  afternoon  the  air  was  dry.  The  tests  were  begun 
at  6h  16m  and  were  continued  until  4h  33m.  Both  leaf-surfaces  were 
studied.  The  leading  results  are  summarized  in  table  26,  in  which 
the  indices  and  the  time  of  the  observations  are  given. 


Table  27. — Transpiring  power  of  Rhus  sp.,  October  21. 


Dorsal  surface 
of  leaf. 

Ventral  surface 
of  leaf. 

6h  40m . 

0.113 

0.378 

7  8  . 

.031 

.204 

7  32  . 

.100 

.134 

8  42  . 

.078 

.235 

9  6  . 

.055 

.160 

140 


FEATURES  OF  THE  VEGETATION  OF  THE 


-0.30$. 


-0.2 


The  observations  on  the  transpiring  power  of  Rhus  sp.  at  Whitehill 
were  made  on  October  21.  The  control  slips  did  not  undergo  color 
change  during  the  hours  of  the  tests.  The  leaves  of  this  species  are 
approximately  10  by  25  mm.  in  size,  with  unlike  dorsal  and  ventral 
surfaces,  although  in  appearance  this  is  not  especially  well  marked. 
The  observations  were  made 
between  6h  40m  and  9h  6m.  For  f*0-33 
the  first  twTo  periods  old  leaves 
were  used;  in  the  following  the 
leaves  were  young.  The  final 
tests,  at  9h  6m,  were  made  on 
large  shade  leaves,  with  results 
as  given  in  table  27. 

It  is  evident  that  in  both 
species  the  transpiring  power 
of  the  two  surfaces  of  the  leaves 
is  strikingly  unlike  and  that 
transpiration  is  carried  on 
largely  on  the  ventral  surface. 

The  difference  is  especially 
marked  in  old  leaves.  The 
ratio  in  transpiring  power  be¬ 
tween  the  ventral  and  dorsal 
surface  is  1  to  3,  or  even  greater 
in  Rhus  viminalis. 


Protea  neriifolia. 

Protea  neriifolia  (plate  19a) 
occurs  at  Tweedside,  altitude 
3,291  feet  above  sea-level,  and 
about  13  miles  west  of  Matjes- 
fontein. 

The  shrub  has  upright 
branches  on  which  the  old 
leaves  take  an  upright  position, 
with  the  upper  or  dorsal  sur¬ 
face  facing  the  branch  and  the 
ventral  surface  without  espe¬ 
cial  protection.  The  young 
leaves  stand  out  somewhat 
from  the  branches  that  bear 


-L 


8Dajn.  10 


12 


2  p.m.  4 


8 


l0hpJT! 


Fig.  13. — Average  indices  of  foliar  transpiring 
power  at  2-hour  intervals,  8h  a.  m.  to  10h 
p.  m.  A,  Protea  neriifolia;  B,  Rhus  vimi¬ 
nalis;  C,  Euryops  lateriflorus;  D,  Grewia 
cana;  E,  Gymnosporia  buxifolia. 


them.  The  old  leaves  are  glabrous,  but  the  young  leaves  are  tomen- 
tose.  The  mature  leaves  are  about  2.8  by  8.3  cm.  in  size  and  the  two 
leaf-surfaces  are  not  greatly  unlike  in  appearance. 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


141 


Observations  on  the  transpiring  power  of  Protea  were  made  October 
23  between  llh  20ra  and  3h  20ra.  The  day  was  clear  and  the  air  was 
dry.  There  was  no  wind.  Three  leaves  were  mainly  used.  Of 
these  No.  1  was  20  by  63  mm.  and  was  situated  immediately  below  the 
tip  of  the  branch.  Leaf  No.  2  measured  18  by  67  mm.  and  was  10  cm. 
from  the  branch-tip.  And  leaf  No.  3  was  3  by  19  cm.  in  size  and  had 
about  the  same  relative  position  as  No.  2.  In  addition  to  these  an 
old  leaf,  which  was  50  cm.  from  the  tip  and  which  measured  26  by 
83  mm.,  was  used  and  also  a  young  leaf  which  was  14  by  42  mm.  in  size. 
Table  28  is  a  summary  of  the  leading  results  of  the  observations. 


Table  28. — Transpiring  power  of  Protea  neriifolia. 


Dorsal  leaf- 
surface. 

Ventral  leaf- 
surface. 

12h . 

0.473 

0.107 

Leaf  No.  2. 

12h  25m . 

.302 

Leaf  No.  1. 

Do . 

.268 

Do. 

2h  45m . 

.376 

Leaf  No.  1,  near  base. 

Leaf  No.  1,  near  tip. 

Leaf  No.  1,  near  base. 

Do . 

.187 

Do . 

.125 

Do . 

.094 

Leaf  No.  1,  middle  of  leaf. 

3h  15m . 

.347 

.205 

Old  leaf. 

3  20  . 

.251 

.161 

Young  leaf,  42  mm. 

It  appears  from  the  few  tests  made  that  in  old  leaves  with  the 
upright  growth  habit  the  index  of  the  transpiring  power  of  the  ventral 
and  outer  surface  is  less  than  that  of  the  dorsal  and  protected  surface. 
This  was  also  the  case  with  the  young  leaf.  The  young  leaf  which  was 
studied  at  the  same  time  the  old  leaf  was  used,  3h  15m  to  3h  20m,  had  a 
heavy  tomentum,  while  the  old  leaf  was  glabrous.  The  smaller  index 
of  the  former  is  striking  and  suggests  the  possible  importance  of  the 
tomentum  as  a  protective  covering  to  the  leaf.  On  the  dorsal  surface, 
at  least,  the  index  for  an  area  near  the  base  is  greater  than  for  an  area 
near  the  tip  of  the  leaf.  Finally,  the  high  index  of  transpiring  power 
of  Protea  neriifolia  is  directly  related  to  the  occurrence  of  the  species 
in  relatively  moist  habitats. 

Namib. 

Welwitschia  mirabilis. 

A  single  series  of  tests  on  the  transpiring  power  of  Welwitschia  was 
carried  out  on  June  28,  at  which  time  also  the  transpiring  power  of  a 
broad-leaved  legume,  Bauhinia,  which  was  growing  not  far  distant, 
was  observed.  Although  thus  limited  in  number,  the  tests  should  be 
of  a  certain  value  because  of  the  interesting  species,  especially  of 
Welwitschia ,  and  of  the  intensely  arid  nature  of  the  habitat. 

The  habitat  of  Welwitschia  is  about  50  km.  east  from  Swakopmund 
and  about  8  km.  south  of  the  Swakop  River.  It  appears  to  be  a  wide 


142 


FEATURES  OF  THE  VEGETATION  OF  THE 


wash  which  inclines  gently  toward  the  north.  Here  and  there  in  the 
slope  are  inconspicuous  channels  which  would  escape  attention  but  for 
the  fact  they  carry  narrow  lines  of  low  shrubs,  which  are  not  numerous. 
For  the  most  part  the  surface  of  the  ground  is  quite  devoid  of  vegeta¬ 
tion.  Among  the  plants  to  be  found  in  these  channels  are  species 
of  Arthrcerua,  Asclepias,  Bauhinia,  and  Zygophyllum.  Welwitschia, 
of  which  5  scattering  representatives  were  seen,  occurs  on  the  plain 
and  without  reference  to  the  drainage-channels;  of  these,  3  were  pos¬ 
sibly  2  mm.  from  tip  to  tip,  2  were  smaller,  and  none  of  them  projected 
above  the  surface  of  the  ground  to  a  height  exceeding  about  15  cm. 

The  specimen  studied  was  one  that  bore  cones  (plate  2).  The 
leaves  were  about  1  meter  in  length.  The  leaf-tips  were  bleached  to  a 
gray  color  and  had  been  whipped  into  strands  by  the  wind,  but  the 
central  and  basal  portion  was  whole  and  of  a  bright  grass  green. 
There  appeared  to  be  no  pubescence  on  the  leaves,  which,  however,  are 
provided  with  a  double  epidermis  and  with  deeply  sunken  stomata. 
The  tests  of  the  transpiration  power  of  Welwitschia  were  carried  out 
about  9  o’clock  in  the  morning.  The  day  was  clear  and  there  was  no 
unusual  wind.  The  temperature  of  the  air  was  23°  C.  The  upper 
leaf-surface,  near  the  base,  w^as  used.  The  average  reaction- time  of 
several  tests  was  120  seconds,  giving  an  index  of  0.1383. 

Bauhinia  marlothii. 

Among  the  shrubs  growing  near  Welwitschia,  as  above  remarked, 
was  Bauhinia  marlothii,  which  was  conspicuous  from  the  fairly  rela¬ 
tively  large  size  of  the  leaflets,  which  measured  2  by  3  cm.,  more  or  less. 
Tests  carried  out  on  Bauhinia  showed  the  same  reaction- time  as 
Welwitschia,  that  is,  120  seconds,  giving  the  index  also  as  0.1383. 

Summary. 

Although  the  studies  on  the  transpiring  power  of  the  species  used 
are  at  best  fragmentary,  in  part  from  the  small  number  and  in  part 
from  the  fact  that  the  work  was  confined  to  the  winter  and  early 
spring,  nevertheless  they  offer  suggestions  of  some  interest  and  indicate 
that  further  investigations  along  the  same  lines  should  bring  fruitful 
results. 

It  was  early  observed  that  under  certain  conditions  of  the  relative 
humidity  of  the  air,  as  when  it  was  fairly  high,  or,  on  the  other  hand, 
fairly  low,  there  was  often  a  disturbing  parallel  in  the  reaction-time 
between  the  hygroscopic  papers  attached  to  the  plant  and  similar 
papers  not  so  attached.  This  led  to  the  use  of  control  slips,  by  which 
it  was  possible  to  detect  the  direct  influence  of  the  humidity  of  the  air, 
if  any,  to  the  end  that  questionable  results  might  be  discarded. 

To  a  certain  and  possibly  limited  extent,  and  possibly  limited  to 
certain  types  of  perennials,  there  may  be  a  real  parallel  between  the 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


143 


relative  humidity  of  the  air  and  the  transpiring  power  of  plants. 
Bakke,  for  example,  has  stated  that  “a  rise  in  evaporation  will  give 
an  increase  in  the  transpiring  power/’  1  that  is,  in  the  species  used  by 
him.  Such  was  thought  to  be  probably  true  in  some  of  the  Karroo 
plants  examined,  particularly  in  Grewia  cana,  Gymnosporia  buxifolia , 
and  Rhus  viminalis ;  but  whether  the  principle  applies  to  Karroo  species 
with  smaller  transpiring  power  remains  to  be  determined. 

In  general  the  older  leaves  appear  to  have  a  higher  index  of  trans¬ 
piring  power  than  young  leaves.  In  Protea  neriifolia,  where  such  was 
observed  to  be  the  case,  the  young  leaves  are  heavily  tomentose. 
In  an  introduced  Eucalyptus  there  was  found  to  be  a  difference  in  the 
index  of  mature  leaves,  which  depended  on  the  leaf-type.  Elongated 
leaves,  on  old  wood,  have  a  lower  index  than  mature  round  leaves  on 
young  wood. 

In  Grewia,  Euclea,  and  Rhus  the  two  leaf-surfaces  have  unlike 
transpiring  power.  The  index  is  always  larger  for  the  ventral  surface. 
Moreover,  the  course  of  transpiring  power  of  the  dorsal  surface  appears 
to  fairly  parallel  the  variation  in  the  humidity  of  the  air,  which,  indeed, 
may  be  the  case,  for  the  reason  that  stomata  are  absent  in  these  species 
from  the  dorsal  leaf-surface. 

In  Protea  the  leaves  assume  an  upright  position,  with  the  ventral 
surface  facing  outward.  In  this  position  the  dorsal  surface  is  rela¬ 
tively  well  protected,  while  the  ventral  surface  is  quite  exposed.  A 
somewhat  similar  leaf  position  is  to  be  found  in  Euryops  lateriflorus, 
although  in  this  case  it  is  not  so  well  marked.  In  Rhus  viminalis 
the  leaves  do  not  appear  to  have  a  fixed  position.  Horizontality  of 
the  leaves  of  Grewia  cana  is  the  rule.  With  the  apparent  exception 
of  Rhus,  therefore,  there  is  a  striking  relation  between  leaf  orientation 
as  regards  light  and  other  external  features,  and  the  surface  with  the 
higher  transpiring  power. 

In  studying  the  transpiring  power  of  relatively  large  leaves,  such 
especially  as  those  of  Protea  neriifolia,  it  was  found  that  to  a  certain 
extent  the  index  varied  with  the  position  of  the  clip  on  its  surface. 
Further,  in  other  species,  especially  in  Grewia  cana,  successive  readings 
made  a  few  minutes  apart  gave  unlike  results,  but,  however,  results 
'which  were  in  a  certain  way  consistent.  Thus,  in  one  instance,  two 
successive  readings  gave  reaction- times  of  100  and  180  seconds,  another 
set  of  two  readings  gave  240  and  270  seconds,  and  a  third  set,  in  which 
the  same  leaf  area  was  carefully  used,  gave  90  as  the  first,  and  4  seconds 
later  150  seconds  as  the  reaction- time.  Such  results  indicate  that  in 
some  manner  the  presence  of  the  closely  applied  glass  clips  disturb 
the  series  of  events  which  accompany  or  cause  water  movements  within 
the  leaf  and  its  final  release  to  the  surrounding  atmospheric  air.  What¬ 
ever  may  be  the  direct  effect  on  these  physiological  features,  it  is  clear 


1  Determination  of  wilting.  A.  L.  Bakke.  Bot.  Gaz.,  vol.  66,  p.  105,  1918. 


144 


FEATURES  OF  THE  VEGETATION  OF  THE 


that  in  the  first  place  the  glass  immediately  operates  to  modify  the 
quality  and  the  quantity  of  light  which  enters  the  leaf,  and  it  operates 
to  exclude  completely  the  saturation  deficit  of  the  air  as  a  factor  in 
removing  water-vapor  from  the  leaf-surface,  and  thus  in  modifying 
the  rate  of  gaseous  movements  in  the  intercellular  leaf-spaces.  The 
acidity  of  the  guard-cells  of  the  stomata  may  be  changed  through  the 
modification  of  the  light  which  strikes  them  and  an  alteration  of  their 
capacity  for  the  absorption  or  excretion  of  water  may  follow  in  its 
wake.1  The  effect  of  the  clips  is  apparently  cumulative  and  the  direc¬ 
tion  of  the  reactions  are  not  easily  or  quickly  changed. 

The  average  indices  of  the  course  of  the  transpiring  power  are  given 
in  table  29,  and  are  graphically  shown  in  figures  9,  10,  and  11.  Al¬ 
though  the  transpiring  power  was  not  studied  to  any  extent  during  the 
hours  of  darkness,  the  curves  suggest  the  possible  daily  course.  For 


Table  29. — Average  indices  showing  the  daily  course  of  the  transpiring  power. 


6  to  8h. 

8  to  10h. 

10  to  12h. 

12  to  2h. 

2  to  4h. 

4  to  6h. 

6  to  8h. 

8  to  10h. 

Aloe  schlechteri .  .  . 

0.106 

0.098 

0.032 

0.024 

Aloe  striata . 

.052 

.020 

.011 

0.014 

.012 

Cotyledon  corns- 

cans . 

.013 

.009 

.008 

.022 

0.009 

.012 

0.012 

0.016 

Cotyledon  panicu- 

lata . 

.029 

.013 

.010 

.008 

.010 

.012 

Eucalyptus  globu- 

lus? . 

.082 

.058 

.170 

.143 

.155 

Euclea  undulata . .  . 

.049 

Euryops  lateriflo- 

rus . 

.133 

.039 

.032 

.023 

.030 

.015 

Gasteria  disticha .  . 

.070 

.052 

.022 

.019 

.010 

Grewia  cana . 

.120 

.213 

.145 

.207 

.115 

Gymnosporia  bux- 

ifolia . 

.106 

.111 

.212 

.125 

.081 

Massonia  latifolia. . 

.017 

.067 

.099 

.094 

.069 

Protea  neriifolia .  .  . 

.333 

.305 

Rhus  viminalis .... 

.132 

.194 

.101 

succulents  the  highest  index  is  apparently  at  night  or  early  morning. 
It  falls  early  in  the  forenoon  and  is  least  at  about  midday  or  early  after¬ 
noon.  In  the  case  of  the  sderophylls,  the  highest  index  of  transpiring 
power  is  apparently  early  in  the  morning,  although  in  the  case  of 
Grewia  it  is  also  high  in  midafternoon.  In  Euryops  it  is  highest  at 
about  sunrise,  when  it  falls  quiekfy.  The  possibly  daily  increase  in  the 
transpiring  power  of  succulents  at  night,  and  the  general  daily  course 
of  the  index  for  sderophylls,  appear  to  be  similar  to  analogous  forms 
as  determined  in  America.2 

A  striking  feature  of  the  transpiring  power  of  the  species  studied 
is  the  low  maximum  as  well  as  the  low  minimum.  The  highest  maxi- 

1  Physiology  of  stomata  of  Rumex  patientia.  J.  D.  Sayer.  Science,  vol.  57,  p.  205,  1923. 

2  See  references  in  Die  Transpiration  der  Pfianzen,  Burgerstein,  2  Th.,  1920. 


1 


MOKE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


145 


mum  index  was  0.472,  for  Protea  neriifolia,  which  is  not  greatly  above 
that  of  xerophytes,  according  to  Bakke.1  The  highest  for  Karroo 
plants  proper  was  0.378,  which  was  noted  in  Rhus  viminalis.  On  the 
other  hand,  the  maximum  as  determined  might  be  extremely  low,  as, 
for  example,  wTas  found  to  be  the  case  in  Cotyledon  coruscans ,  where  it 
was  determined  to  be  0.016. 

A  very  low  minimum  was  found  in  the  succulents,  especially  in 
Cotyledon  coruscans ,  where  almost  failure  of  transpiration  was  demo- 
strated.  The  ratio  between  maximum  and  minimum  indices  in 
C.  paniculata  was  found  to  be  106:  1,  as  opposed  to  between  3  and  10 
to  1  as  in  most  of  the  cases  noted. 

The  tests  during  one  morning  on  the  transpiring  power  of  Bauhinia 
marlothii  and  Welwitschia  mirabilis  indicated  that  although  growing 
under  extremely  arid  conditions,  both  species  have  surprisingly  high 
transpiration  power.  Further,  the  fairly  high  indices,  for  plants  in  a 
so  arid  region,  were  of  the  type  characteristic  of  species  without  water- 
balance,  or  at  least  of  sclerophylls  rather  than  of  succulents.  This 
conclusion  is  advanced  tentatively,  inasmuch  as  further  observations 
must  needs  be  made  in  order  to  definitely  establish  it. 

GENERALIZED  SUMMARY. 

In  the  main  part  of  this  paper  summaries  have  been  prepared  for 
each  subhead,  and  to  such  the  reader  is  referred  for  summarized  details 
of  particular  subjects.  It  is  proposed,  as  to  the  few  following  para¬ 
graphs,  to  present  some  more  general  features  relative  to  both  the 
environment  and  to  the  plants,  in  which  the  general  student  of  the 
arid  portions  of  southern  Africa  may  find  interest. 

The  latitudinal  situation  of  the  regions  of  the  subcontinent  in 
which  there  is  little  rain  corresponds  fairly  well  to  similar  regions 
of  Australia  and  of  South  America.  Thus  the  latitude  of  Lake 
Eyre  of  South  Australia  is  about  that  of  the  Central  and  Upper 
Karroos  and  of  the  southern  extension  of  the  arid  region  of  the 
extreme  west.  Thus  the  situation  of  the  arid  regions  in  the  south 
temperate  zone  is  such  as  to  place  them  under  the  influence  both 
of  the  southeast  trades  and  of  the  prevailing  westerlies,  with  impor¬ 
tant  resulting  effects  on  the  seasonal  distribution  of  the  rainfall. 
Such  influences,  however,  are  modified  by  others  in  southern  Africa, 
to  which  the  general  climate  owes  much  of  its  character.  Thus  there 
is  the  relation  to  the  seas  and  to  the  general  relief  which  are  also  most 
important.  To  the  former  is  due  the  drop  towards  the  south  of  the 
isothermal  lines  leading  westward,  as  well  as  the  fairly  low  tempera¬ 
tures  of  the  north.  And  there  are  direct  effects  on  the  precipitation  as 
well.  Thus  the  air  above  the  warm  equatorial  ocean  current  which  is 

1  Studies  on  the  transpiring  power  of  plants,  as  indicated  by  the  method  of  standardized 
hygrometric  paper.  A.  L.  Bakke.  Journ.  Ecology,  vol.  2,  1914. 


146 


FEATURES  OF  THE  VEGETATION  OF  THE 


off  the  east  shore  is  heavily  moisture-laden  as  it  drifts  over  the  land  in 
summer,  bringing  rains  not  only  to  the  mountains  and  the  plateaus 
near  the  coast,  but  into  the  far  interior  as  well,  and  reaching  even  into 
Southwest  Africa.  In  general,  such  precipitation  decreases  toward  the 
west  and  may  be  little  in  the  interior  valleys,  which  are  in  the  rain- 
shadow  of  the  mountains.  Along  the  western  shore,  on  the  other 
hand,  the  ocean  current  is  from  the  south,  so  that  the  cold  air  above 
it  holds  relatively  small  amount  of  vapor.  In  the  extreme  southwest 
there  are  copious  rains  in  winter,  but  in  lower  latitudes  or  the  interior 
the  winters  are  fairly  rainless.  Between  the  regions  of  summer  and 
of  winter  rains,  in  southern  Africa  and  also  in  Australia,  lies  an  inter¬ 
mediate  belt  where  the  rainfall  is  at  once  uncertain,  not  only  as  to 
seasonal  distribution,  but  also  as  to  amount.  This  is  the  arid  belt, 
extending  in  a  general  southwest-southeast  direction,  in  which  are  to 
be  found  the  Karroos,  as  well  as  the  Namaqualands,  and  in  the  latter 
is  included  the  Namib,  the  most  arid  portion  of  the  subcontinent. 
But  in  much  of  the  Karroo  province,  especially  the  eastern  portion, 
the  most  of  the  rain  is  in  the  warm  seasons.  The  seasonal  distribution 
of  the  rain  is  of  importance  from  the  fact  that  the  rainless  seasons, 
having  unlike  temperature  relations,  have  corresponding  unequal 
values  as  regards  the  evaporation.  As  environmental  factors,  such  con¬ 
ditions  have  well-recognized  vegetational  effects.  Where  the  dry 
periods  correspond  with  the  seasons  of  higher  temperatures,  the 
perennial  flora  is  markedly  sclerophytic ;  chaparral  or  macchia  is  char¬ 
acteristic  of  the  Cape.  But  where  the  dry  period  is  in  the  cool  seasons, 
other  plant  types  prevail,  along  which  are  those  typical  of  the  Karroos, 
succulents,  generally  of  small  stature.  In  this  instance,  however,  as 
will  be  commented  on  later,  the  causal  relation  may  lie  rather  with 
the  coincidence  of  sufficient  moisture  and  appropriate  temperatures. 
So  far  as  the  actual  amount  of  precipitation  in  the  drier  portions  of 
southern  Africa  is  concerned,  it  can  be  said  to  range  from  less  than  1 
inch  in  the  western  Namib  to  about  15  inches  in  the  lesser  Karroos 
and  in  the  eastern  portion  of  the  Central  Karroo.  Not  all  of  the  pre¬ 
cipitation,  however,  finds  its  way  to  the  water  reservoirs  of  the  ground 
or  is  of  more  direct  use  to  plants.  So,  for  example,  a  large  rainfall 
within  a  short  time  may  be  mostly  lost  by  run-off;  and,  on  the  other 
hand,  an  amount  of  rain  so  small  as  not  to  enter  the  soil  appreciably 
is  of  little  direct  benefit.  Such  non-effective  rainfall  is  usually  rela¬ 
tively  large  in  amount  where  the  rainfall  is  little  and  thus  may  be 
an  environmental  feature  of  importance.  Non-effective  rainfall  is 
defined  as  being  less  than  0.15  inch,  occurring  in  a  single  stormy  period. 
For  several  stations  examined  in  this  particular,  it  was  found  that  the 
mean  annual  non-effective  precipitation  varied  from  6  to  16  per  cent 
of  the  entire  amount  recorded.  The  maximum  was  found  to  be  29 
per  cent.  Fairly  high  percentages  of  non-effective  rainfall  are  charac- 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


147 


teristic  for  stations  in  the  Karroos.  The  percentages  of  non-effective 
rainfall  given  above  are,  however,  not  especially  high.  Somewhat 
higher  maximum  non-effective  precipitation  has  been  determined  for 
the  Lake  Eyre  region,  South  Australia,  for  example,  where  so  much  as 
43  per  cent  of  the  rainfall  of  one  year  should  be  so  classed. 

In  portions  of  southern  Africa  where  the  summer  type  of  rainfall 
prevails,  the  seasonal  course  of  evaporation  varies  relatively  little,  and 
the  largest  amount  may  be  in  spring,  or  at  least  other  than  in  summer; 
but  where  the  rains  are  in  winter  the  evaporation  is  not  only  relatively 
large,  but  it  is  also  more  variable  as  between  seasons.  In  this  case 
that  of  summer  is  the  most.  In  order  to  somewhat  more  closely 
determine  the  evaporation  at  certain  representative  stations,  atmom- 
etry  was  introduced  in  southern  Africa.  Although  the  preliminary 
results  are  not  sufficient  for  generalization,  they,  however,  indicate 
something  of  the  possibilities  of  studying  evaporation  in  southern 
Africa  in  this  manner.  A  convenient  and  useful  means  of  expressing 
aridity  is  to  find  the  ratio  between  the  amount  of  rainfall  for  a  given 
period  and  divide  this  by  the  amount  of  evaporation  for  the  same 
period.  This  is  the  index  of  aridity  and  gives  a  ready  method  of  com¬ 
paring  the  aridity  of  different  stations  as  well  as  the  seasonal  varia¬ 
tion  at  one  and  the  same  station,  and  has  been  much  employed  in  other 
countries  for  these  purposes.  Mention  has  already  been  made  of  the 
relatively  high  evaporation-rate  in  summer  at  stations  where  the  rains 
occur  in  winter.  An  illustration  is  afforded  by  the  results  at  the 
National  Botanic  Gardens  near  Cape  Town.  In  1921-22  the  follow¬ 
ing  indices  were  obtained  for  winter  and  summer,  respectively,  namely: 
0.0403  and  0.0005.  As  to  differences  when  shorter  time  periods  are 
concerned,  the  results  at  Irene  are  of  interest.  At  this  station  having 
summer  rains,  the  July  index  was  found  to  be  0.0007,  while  that  of  the 
following  October  (midspring)  was  0.0038.  Finally,  extremely  low 
ratios  were  obtained  in  summer  where  there  were  no  rains,  as  in  por¬ 
tions  of  the  Central  Karroo.  At  Matjesfontein,  in  January  1922,  for 
example,  the  index  was  0.000016!  An  attempt,  which  was  wrecked 
by  miscreants,  was  made  to  determine  the  relative  aridity  of  different 
faces  of  a  kopje  in  the  Karroo.  Comparative  results  showed,  however, 
that  the  northern  face  of  the  kopje  was  markedly  more  arid,  as  re¬ 
vealed  by  the  atmometer,  than  the  face  opposite,  and  this  conclusion 
was  carried  out  by  the  observed  differences  in  the  flora.  With  the 
systematic  study  of  evaporation  as  shown  by  the  atmometer,  which  is 
in  progress  in  southern  Africa,  observations  of  this  kind  will  be  greatly 
multiplied  and  evaporation  as  an  ecological  factor  in  southern  Africa 
will  be  more  thoroughly  defined  than  is  possible  at  present. 

In  traversing  the  regions  with  marked  alterations  of  rainless  and 
of  rainy  periods,  or  the  intermediate  regions  where  the  rainfall  is  little 
and  irregular  as  to  time,  the  visitor  sees  a  great  variety  of  perennial 


148 


FEATURES  OF  THE  VEGETATION  OF  THE 


vegetation,  even  in  the  arid  and  semi-arid  Karroos  and  in  the  regions 
north  and  northwest.  Except  along  the  streamways  in  the  Karroos, 
in  the  savannah-forest  west  of  Windhoek  in  Southwest  Africa,  and  in 
the  vicinity  of  Messina  in  the  Low  Veld,  the  writer  encountered  few 
trees,  that  is  to  say,  in  regions  such  as  are  especially  considered  in  this 
paper.  For  the  most  part,  the  perennials  that  attract  attention  are 
small  shrubs,  but  the  variety  of  these  is  surprising.  It  may,  very 
possibly,  convey  a  right  impression  if  one  should  state  that  toward 
the  region  of  winter  rains  selerophylls  appear  to  dominate,  with  little 
suggestion  of  either  deciduous  types  or  of  those  that  are  succulent; 
that  in  the  intermediate  region,  especially  the  Karroos,  small  succu¬ 
lents  compose  an  important  proportion  of  the  perennial  flora;  but  that 
where  the  summer  rains  prevail  shrubs  are  not  so  abundant;  here 
grasses  come  in,  and  in  certain  areas  of  large  rainfall  there  are  succu¬ 
lents  of  large  size.  In  the  Karroos  and  in  the  region  of  predominant 
summer  rains,  both  deciduous  and  evergreen  types  are  to  be  found. 
However,  to  give  an  adequate  characterization  of  the  flora  in  relation 
to  the  rainfall  regions  beyond,  such  general  statements  may  not  here 
be  possible,  and  is  at  least  not  necessary  for  the  present  purposes. 
A  leading  point  to  be  made  is  that  there  are  tendencies  toward  one  or 
another  type  of  development,  but  that  as  a  whole  the  result  is  very 
confusing.  This  is  possibly  to  be  expected  from  the  very  wide  extent 
of  the  dry  habitats.  Mesembryanthemum  spinosum,  with  small  but 
fleshy  leaves  and  with  small  spines,  is  an  example  of  this  mixed  type 
of  development.  The  character  of  the  roots  of  this  species  (plate  26), 
which  are  essentially  superficial,  indicate  that  it  should  probably  be 
classed  among  the  succulents.  As  indicated  in  figure  7,  they  extend 
far  in  an  east-west  direction  and  are  closely  connected,  on  the  east 
and  south,  with  regions  which  also  have  a  diverse  climate,  but  one 
in  which  the  rainfall  is  good.  According  to  the  work  of  Bews  and  of 
others,  the  relationship  of  the  mesophytic  flora  of  the  latter  regions  and 
that  of  the  flora  of  the  dry  habitats  is  close.  This  feature  has  possi¬ 
bilities  which  will  be  commented  on  in  another  connection  below,  but 
in  this  place  I  wish  merely  to  point  to  the  fact  of  climatic  diversity, 
with  especial  regard  to  the  dry  habitats,  and  the  further  and  pre¬ 
sumably  related  fact  of  diversity  in  flora.  Not  all  of  these  conditions 
of  climate  and  of  flora  are  sharply  cut.  There  is  more  or  less  inter¬ 
relationship  and  dovetailing  in  both  particulars.  Thus,  it  is  to  be 
noted  that  in  portions  of  the  Karroos  there  is  some  rainfall  in  summer, 
although  there  may  also  be  winter  rains. 

It  does  not  appear  to  be  unlikely,  from  what  is  known  of  the  depend¬ 
ency  of  succulents  on  rains  during  the  warm  seasons,  that  the  presence 
in  the  Karroos  of  plants  of  this  type  is  to  be  traced  immediately  to  the 
rainfall  of  the  warm  seasons.  Where  the  summer  rains  are  copious 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


149 


the  species  with  water-balance  may  be  of  large  size.  Thus,  one  may 
meet  large  Euphorbias  with  arboreal  habit  of  growth  (plate  5c),  or  shrubs 
with  much  shortened  and  bulb-like  stems  which  rest  wholly  on  the  sur¬ 
face  of  the  ground  (plate  6c),  or  trees  with  massive  stems  and  branches 
(plate  5a,  5b)  which  may  perhaps  be  rightly  classed  with  species  having 
water-storage  capacity.  In  the  dry  habitats,  however,  where  the 
rains  of  summer  are  not  so  abundant,  the  succulents  are  usually  not 
large,  but  yet  they  are  extremely  diverse  as  to  growth-forms.  It  will 
suffice  to  point  to  those  that  are  cushion-shaped  (plate  11),  others 
which  are  very  small  (plate  29b),  and  to  the  leaf  succulents  of  different 
form  and  size  (plates  11,  17,  and  18),  as  well  as  to  such  stem  succu¬ 
lents  as  certain  Euphorbias  (plates  8b;  13;  25c),  and  to  the  boter- 
boom,  Cotyledon  paniculata  (plate  18).  The  species  last  given  is  of 
particular  interest  in  that  it  not  only  is  the  largest  succulent  of  the 
Karroos,  but  that  it  occurs  typically  in  regions  having  relatively  good 
rainfall,  of  which  much  is  in  winter.  It  appears,  however,  that  the 
species  is  vegetatively  active  during  the  fall  and  winter  and  that  in 
late  spring  and  in  summer  the  leaves  drop  away,  leaving  the  photo¬ 
synthetic  activities  to  be  carried  on  by  the  green  and  fleshy  branches. 
During  the  warm  season,  also,  the  boterboom  produces  flowers  and 
fruits.  The  species  appears  from  this  to  be  vegetatively  reactive  to 
rather  low  temperatures.  Even  so  it  may  be  questioned  whether 
without  the  rains  of  summer  it  would  renew  growth  so  actively  in 
spring.  The  roots  are  superficially  placed,  as  is  the  rule  in  plants  of 
this  class,  and,  for  this  as  the  leading  reason,  their  growth  may  require 
fairly  warm  soil,  as  well  as  one  with  a  suitable  amount  of  moisture. 
But  the  temperature  of  spring  probably  meets  this  requirement. 
Further  than  this,  the  transpiring  power  of  the  leaves  of  the  boterboom 
is  relatively  high,  so  that  with  no  more  adequate  regulatory  mechan¬ 
ism  of  this  function  than  they  appear  to  possess,  the  plant  would  not 
withstand  the  high  evaporation-rates  of  summer  without  injury. 
There  does  not  appear,  however,  to  be  any  reason  to  suppose  that  the 
roots  may  not  be  active  in  moist  soil  in  summer,  and  that  being 
capable  of  absorbing  water,  they  may  not  supply  the  water  needs  of  the 
species  of  that  season. 

Comment  has  been  made  to  the  effect  that  mucilages  in  plants  are 
organized  under  relatively  moist  conditions,  while  cell-wall  material 
is  formed  where  the  conditions  are  especially  arid.  These  physio¬ 
logical  circumstances  probably  represent  tendencies  in  development 
and  stand  for  very  fundamental  differences  in  metabolism.  Thus,  in 
dry  regions,  spininess  is  often  a  marked  characteristic,  with  the  physio¬ 
logical  process  referred  to  as  the  probable  immediate  cause,  although  the 
species  may  have  fairly  marked  water  requirements.  Acacia  karroo 
(plate  14)  is  most  striking  in  this  regard.  But  it  is  to  be  noted  that 


150 


FEATURES  OF  THE  VEGETATION  OF  THE 


one  and  the  same  species  may  be  at  once  succulent  and  also  spiniferous, 
although  the  development  in  either  direction  can  not  be  said  to  be 
extreme. 

The  non-succulents,  both  evergreen  and  deciduous,  of  the  dry 
regions  of  southern  Africa  are  likewise  exceedingly  various  and  present 
features  of  interest;  and,  in  addition,  such  regions  are  also  noteworthy 
for  what  they  do  not  possess,  but  might  be  expected  to  possess  of 
plants  of  such  types,  and  also  the  absence  of  certain  organs  found  in 
analogous  regions  elsewhere  is  at  once  curious  and  striking.  Thus 
species  of  the  genus  Acacia,  as  well  as  of  the  Proteacese,  which  are  to  be 
found  under  conditions  of  relatively  great  aridity  in  South  Australia, 
where  they  are  either  large  shrubs  or  small  trees,  and  where  they  may 
stray  away  from  water-courses,  are,  in  southern  Africa,  either  wholly 
wanting  in  the  dry  regions  or  confined  to  the  banks  of  streams  or  to 
bottoms  adjacent  to  them.  In  both  continents  the  Proteacese  are 
evergreen,  and  in  Australia  the  same  is  true  of  species  of  Acacia, 
which  are  numerous.  However,  in  this  genus  the  evergreen  quality  is 
occasioned,  as  is  well  known,  by  the  development  of  phyllodia  which 
vary  greatly  in  size  and  in  form,  and  such  are  wanting  in  the  species  of 
southern  Africa.  It  is  of  little  use  to  speculate  as  to  the  reasons  for 
the  absence  in  the  one  case  and  the  presence  in  the  other  of  species  or 
structures.  It  can  be  pointed  out,  however,  that  in  the  phyllodium 
of  such  Australian  species  as  Acacia  linophylla,  of  the  intensely  arid 
region  west  of  Lake  Eyre,  the  proportion  of  living  cells  of  the  “ leaves” 
is  relatively  small,  and  per  contra,  that  of  non-living  material  is  rela¬ 
tively  very  great.  Such  being  the  case,  the  water  requirement  is 
small  at  all  times  and  the  species  is  in  position  to  produce  foods  photo- 
synthetically  also  at  all  times. 

With  the  relatively  small  size  of  the  phyllodia  of  this  species  and 
the  small  number  on  a  plant,  even  of  good  size,  it  can  be  seen  that  it  is 
wonderfully  adjusted  to  an  arid  environment;  and  it  is  to  be  doubted 
whether  species  of  the  same  size  and  habit  of  growth,  but  with  de¬ 
ciduous  leaves  or  leaflets,  even  small  in  size,  would  survive  in  the  habitat 
referred  to.  Thus  the  tendency  of  species  to  organize  anhydrides 
under  conditions  of  aridity  works  in  the  direction  of  also  better  ad¬ 
justing  them  for  the  unfavorable  moisture  relations  of  an  arid  habitat, 
and  this  largely  by  insuring  not  only  protection  for  the  living  cells, 
of  which  many  are  engaged  in  carbon  assimilation,  but  also  probably 
to  a  certain  degree  extending  the  possibilities  of  water  storage,  as  well 
as  to  bring  about,  indirectly  to  be  sure,  the  fact  that  a  relatively  small 
proportion  of  the  tissues  can  be  said  to  be  living. 

In  his  rapid  survey  of  the  flora  the  writer  failed  to  see  aphyllous 
species  of  consequence,  aside  from  the  Euphorbias,  although  certain 
forms  with  green  branches  were  seen  to  be  without  leaves  in  winter. 
It  is  possible,  therefore,  that  aphylly  is  not  a  common  occurrence  in 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


151 


the  arid  habitat  of  southern  Africa.  The  perennials  do  exhibit  reduc¬ 
tion  in  leaf-surface,  however,  in  different  ways  and  often  in  remarkable 
degree.  In  this  respect,  however,  they  were  not  especially  different 
from  analogous  plants  in  other  and  similar  dry  habitats.  Beyond 
the  reduction  of  the  external  surface,  especially  of  foliar  organs,  the 
possible  means  of  cutting  down  excessive  evaporation,  by  morpho¬ 
logical  means,  were  found  to  lie  in  the  reduction  of  the  size  of  the  inter¬ 
cellular  spaces,  by  which  the  internal  evaporation  surface  is  reduced, 
the  frequent  limitation  in  amount  of  the  living  tissues,  whether  chloro¬ 
phyll-bearing  or  not,  and  the  coating  of  the  outer  surface  with  waxy 
or  resinous  materials.  So  far  as  these  features  are  concerned,  comment 
need  only  be  made  of  those  last  referred  to.  Frequently,  possibly 
always,  the  secretion  of  wax  or  resin  is  of  ecological  significance.  Thus 
in  Rhus  viminalis,  which  occurs  by  streams  in  the  Karroo,  the  resin¬ 
ous  secretion  is  slight,  but  in  Rhus  sp.,  which  grows  on  low  kopjes  not 
far  distant,  the  amount  of  secretion  of  this  nature  is  very  much  greater 
and  in  fact  is  very  considerable.  When,  however,  it  becomes  exces¬ 
sively  thick,  as  on  the  stems  of  species  of  Sarcocaulon  is  sometimes  the 
case,  it  should  possibly  be  regarded  as  a  physiological  condition  and 
as  such  a  direct  reaction  to  the  arid  environment.  It  does  not  appear 
that  the  leaves  of  these  plants  are  exceptionally  heavily  covered  with 
resin,  but  only  the  stems.  It  should  be  remarked  here  that  trichomes 
of  various  sorts  are  often  found  in  young  leaves,  but  less  often  in 
those  that  are  mature.  In  such  case  the  exposed  surface  may  be 
greatly  increased,  but  owing  to  the  character  of  the  trichomes  they 
may,  however,  operate  to  shield  the  leaf  against  harmful  effects  of  ex¬ 
cessive  illumination  as  well  as  against  a  dangerous  rate  of  water-loss 
from  the  epidermal  cells. 

The  perennials  of  such  arid  habitats  as  those  of  southern  Africa 
often  have  roots  which,  in  the  direction  of  their  development  as  well  as 
in  size  and  in  other  features,  may  be  highly  characteristic.  Thus  it 
appears  that  as  a  rule  succulents  have  the  type  of  root  system  in 
which  none  penetrate  the  ground  deeply,  not  even  excepting  the 
anchoring  roots.  Those  of  non-succulents,  on  the  other  hand,  may 
exhibit  fairly  wide  diversity  in  this  regard,  and  with  respect  to  root 
development  may  be  quite  comparable  to  similar  plants  in  dry  habi¬ 
tats  elsewhere.  In  certain  species,  as  Lycium  sp.,  where  there  are 
superficial  roots  as  well  as  those  which  penetrate  deeply,  the  nature  of 
the  soil  permitting  this,  shoots  may  spring  from  those  which  lie  close 
to  the  surface  of  the  ground.  How  common  such  vegetative  means 
of  reproduction  may  be  in  southern  Africa  was  not  learned.  In  other 
arid  regions,  as,  for  example,  in  South  Australia,  it  was  found  in  several 
species,  and  it  is  also  known  to  occur  in  semi-arid  portions  of  south¬ 
western  United  States.  Owing  to  the  inherent  difficulties  of  becoming 
established  in  an  arid  habitat,  many  of  which  are  overcome  by  such 


152 


FEATURES  OF  THE  VEGETATION  OF  THE 


means,  it  is  rather  surprising  that  it  is  not  more  commonly  met.  The 
effect  will  be  seen  to  be  somewhat  different  than  the  renewal  of  an 
individual  through  the  upspringing  of  shoots  at  the  bases  of  the  stems 
or  branches.  In  the  former  case  new  territory  is  invaded,  while  at 
the  same  time  that  previously  occupied  is  yet  retained. 

Another  feature  which  should  be  mentioned  is  the  occurrence  of 
slender  rootlets  in  groups  in  the  soil  horizon  most  commonly  wetted  by 
the  rains.  These  appear  to  be  formed  each  season  with  the  moistening 
of  the  soil  and  to  dry  up  and  die  when  the  upper  soil  becomes  im¬ 
possibly  dry.  Such  “  deciduous’ ?  rootlets  are  seemingly  peculiar  to 
perennials  of  an  arid  habitat  and  appear  not  to  be  formed  in  more  moist 
situations.  Mention  was  made  in  the  preceding  paragraph  to  the 
effect  that  in  succulents  the  roots  are  not  deeply  placed.  The  state¬ 
ment  requires  qualifying.  The  writer  did  not  see  the  roots  of  the  tree¬ 
like  Euphorbias ,  which  do  not  occur  in  the  arid  regions,  but  he  sup¬ 
poses  from  analogy  that  they  possess  anchoring  roots  which  pro¬ 
vide  a  portion  at  least  of  the  support  of  the  plant  and  which  may 
penetrate  the  ground  to  a  certain  depth.  However,  in  the  geophytic 
species,  as  E.  multiceps,  where  anchorage  of  the  sort  referred  to  is  not 
required,  the  root-system  is  dominated  by  a  tap-root  which  reaches 
fairly  deeply.  In  this  case  the  root  is  highly  specialized  and  of  the 
obligate  type.  No  superficial  secondary  roots  of  importance  are 
organized.  It  is  a  striking  exception  to  the  rule  for  root  formation  in 
succulents. 

The  chlorophyll-bearing  organs  of  perennials  are  the  portions  of  the 
plant  most  directly  affected  by  changes  of  whatever  sort  in  the  subaerial 
environment.  In  a  dry  habitat,  such  as  the  Karroos  or  the  Namib, 
it  is  not  surprising  that  the  structure  of  such  organs  exhibit  to  a  degree 
the  results  of  the  reaction  of  the  leaf,  for  example,  to  the  impinging 
environment,  but  it  is  of  interest  to  note  the  presence  in  the  foliar 
organs  of  useless  (?)  structures  common  to  such  members  of  the  family 
as  may  be  living  under  mesophytic  conditions.  By  contrasting  the 
essentials  in  the  inner  morphology  of  leaves  of  xerophytic  and  meso¬ 
phytic  members  of  the  same  family,  it  seems  possible  to  arrive  at  the 
leading  structural  adjustments  wrought  in  response  to  living  in  the  arid 
habitat.  This  has  been  done  tentatively  and  in  a  very  limited  way 
in  the  body  of  this  paper,  and  need  not  be  presented  in  detail  in  this 
place.  Certain  of  the  more  interesting  features,  however,  may  be 
alluded  to. 

It  is  pointed  out  that  the  formation  of  a  heavy  outer  epidermal  wall 
occurs  in  all  species  where  there  is  no  covering  of  trichomes.  It 
should  be  added  that  in  the  species  studied  the  inner  wall  of  the 
epidermis  remained  unthickened  and  that  usually  only  the  outer 
portion  of  the  lateral  walls  increased  in  thickness,  although  prominent 
exceptions  to  the  statement  last  made  were  met  with.  Such  conditions 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


153 


would  be  expected,  in  view  of  the  circumstance  that  cell- wall  material 
is  actively  organized  in  conditions  of  aridity  to  which  it  will  be  noted 
the  outer  epidermal  wall  of  such  species  is  always  especially  subject. 
The  result  has  ecological  importance,  but  the  process  is  purely  physi¬ 
ological  and  a  teleological  explanation  is  not  fitting. 

A  frequent,  but  not  universal  character  of  the  xerophytes  examined 
wTas  found  to  be  deep  placing  of  the  stomata  with  respect  to  the  general 
level  of  the  leaf-surface ;  but  this  does  not  entail  the  development  of  a 
novel  character,  from  the  fact  that  it  is  apparently  always  through 
the  formation  of  a  heavy  outer  wall  of  the  epidermis  that  the  stomata 
come  to  have  this  position.  It,  however,  was  noted  in  certain  species 
that  there  is  a  marked  constriction  of  the  entrance  of  the  stomatal  pit, 
which,  in  addition  to  the  outer  vestibule  ridge,  must  operate  to  cut 
down  the  passage  of  gases.  The  epidermis  was  found  to  consist  of  only 
one  layer  of  cells,  except  in  three  species,  of  which  two  only,  and  pos¬ 
sibly  only  one,  Welwitschia,  can  be  said  to  be  markedly  eremophytic. 
As  to  other  structures,  it  will  suffice  to  remark  that  supporting  tissue 
was  not  found  so  abundant  in  the  foliar  organs  of  southern  Africa  as  in 
those  of  South  Australia.  As  to  the  chlorenchyma,  it  need  only  be 
remarked  that  most  of  the  species  have  palisades  on  both  sides  of  the 
leaves,  but  many  leaves  are  not  isosymmetrical,  but  are  dorsi-ventral 
in  their  structural  symmetry.  The  last  remark  applies  to  the  non¬ 
succulents.  In  succulents  the  outer  chlorophyll-bearing  cells  are 
not  pronounced  palisades  and  those  within  are  cuboid,  but  the  chloren¬ 
chyma  is  apparently  alike  on  both  sides  of  the  leaf. 

The  possible  departures  from  structural  features  characteristic 
of  mesophytic  members  of  the  families  to  which  the  species  examined 
belong  will  be  seen  thus  to  be  surprisingly  few  in  number,  although 
often  striking  when  studied  in  detail.  They  consist  very  largely  in 
relatively  small  modifications  of  the  epidermis  and  of  epidermal  organs 
with  a  marked  tendency,  in  non-succulents,  to  the  formation  of  cell- 
wall  material,  as  to  the  inner  morphology,  and  in  a  reduction  of  the 
surface  of  the  transpiring  organs  as  well.  Such  divergences  from  the 
features  of  the  mesophytic  members  of  the  family  are  of  a  consequence 
of  the  order  of  qualitative  differences  and  are  strikingly  small  when  the 
intimate  relationship  of  the  leaf  with  the  marked  characteristics  of  the 
arid  environment  are  taken  into  account.  As  to  morphological  rela¬ 
tions  of  other  portions  of  the  plant,  which  are  not  here  considered, 
even  greater  conservatism  might  be  expected  and  probably  may  be 
found. 

In  a  general  way,  there  appears  to  exist  a  very  definite  relation 
between  the  ability  of  a  xerophyte,  and  reference  is  here  only  made  to 
those  studied  in  this  particular,  to  give  off  water- vapor  and  the  aridity 
of  the  habitat  and  thus  with  the  xerophytism  of  the  species.  Should 
it  be  possible  to  account  for  all  the  water  received  in  the  habitat,  and 


154 


FEATURES  OF  THE  VEGETATION  OF  THE 


in  every  way,  there  might  be  little  exception  to  this  statement.  Inde¬ 
pendent  of  this,  however,  it  still  is  often  true  that  the  transpiring 
power  of  the  leaves  checks  very  closely  with  the  character  of  the  plant 
and  with  that  of  its  environment.  Thus  such  species  as  Protea  nerii- 
folia,  which  does  venture  away  from  fairly  mesophytic  conditions, 
appears  to  have  a  relatively  high  transpiring  power,  while  that  of 
Gymnosporia  buxifolia,  of  the  Karroo,  is  relatively  low,  to  cite  two 
examples.  A  very  low  index  was  determined  for  leaf  succulents  at 
Matjesfontein,  in  certain  of  which  the  fact  of  transpiration  was  on 
occasion  determined  with  difficulty.  Such  forms  were  confined  to 
low  kopjes.  On  the  other  hand,  species  by  the  water-course  showed 
fairly  high  indices  of  transpiring  power.  Regulatory  adjustment,  by 
means  of  which  destructive  loss  of  water  is  avoided,  appeared  to  be 
the  case  in  certain  species  in  which  the  transpiring  power  was  higher 
when  the  air  was  humid  than  when  the  relative  humidity  was  low. 

There  thus  seems  to  be  a  relationship  between  structure,  physio¬ 
logical  reactions,  shoot  and  root  development,  and  the  local  and  general 
distribution  of  the  species.  As  to  the  last,  this  paper  has  little  to  do. 
The  botanists  of  South  Africa  are  actively  engaged  in  following  out  the 
distribution  and  possible  origin  of  the  flora,  including  that  of  the  arid 
habitats,  in  an  intense  and  well-organized  manner.  Some  remarks, 
however,  may  not  be  out  of  place  as  regards  the  occurrence  of  species 
in  such  habitats  as  the  writer  had  under  observation,  which  are  situated 
in  the  western  part  and  the  east-central  part  of  the  Central  Karroo, 
and  a  small  area  of  the  Namib  Desert. 

The  local  distribution  and  occurrence  of  the  woody  plants  have 
some  features  of  interest.  The  larger  species,  as  trees,  may  be  present 
even  in  a  desert  district,  but  if  so  they  are  confined  to  water  channels. 
The  relative  humidity  of  the  air  may  be  low,  but  if  the  roots  are 
provided  with  an  adequate  supply  of  water  a  high  rate  of  evaporation 
appears  not  to  be  inimical  to  many  trees.  But  the  lesser  perennials 
have  greater  range  in  their  distribution.  Where,  as  on  the  Namib, 
the  conditions  of  aridity  are  intense,  shrubs  are  generally  wanting  and 
may  only  be  found  in  shallow  drainage  channels,  if  on  the  higher  plain, 
or  by  the  main  streamways.  In  somewhat  better  watered  regions 
the  shrubs  creep  out  on  the  plain  away  from  the  water-courses  of 
whatever  kind.  They  are  of  good  size  and  abundant  if  the  ground- 
supply  of  water  is  fairly  plentiful,  or  they  may  be  of  good  size  but 
scattering;  or  finally,  they  may  be  relatively  small  but  still  fairly 
abundant  if  soil  or  water  conditions  are  not  both  relatively  favorable. 

The  above  remarks  have  to  do  with  non-succulents  more  par¬ 
ticularly.  If  succulents  are  present,  and  especially  if  they  are  not  of 
large  size,  they  are  often  gathered  in  numbers  at  the  bases  of  the  larger 
non-succulents,  increasing  the  total  population  and  even  doubling  it. 
Such  distributional  characteristics  are  often  encountered  in  the 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


155 


Central  Karroo,  together  with  others  which  need  not  be  taken  up  in 
this  paper. 

The  reactions  and  adjustment  associated  with  the  local  distribution 
of  the  lesser  perennials  are  probably  sufficiently  complex,  and  no 
attempt  will  be  made  to  adequately  account  for  them;  but  there  are 
certain  relationships  which  are  apparently  causal  to  which  allusion 
may  be  made.  Thus,  to  take  up  at  first  the  condition  last  referred  to, 
that  is,  the  relationship  of  succulents  and  of  non-succulents,  it  may 
be  pointed  out  that  the  root-system  of  the  former  is  necessarily  meager 
and  as  a  whole  situated  close  to  the  surface  of  the  ground,  while  that 
of  the  larger  “ protecting”  form  may  be  extensive  and  may  penetrate 
the  ground  deeply,  or  fairly  so.  Such  mutual  accommodation  de¬ 
creases,  if  it  does  not  wholly  do  away  with,  competition  between  the 
roots  of  the  two  types  of  plants  for  room  and  moisture.  But  the 
matter  may  not  end  here. 

It  seems  possible  that  the  succulent  may  possibly  derive  real  pro¬ 
tection  from  excessive  light,  or  evaporation,  by  the  association,  or  that 
there  may  otherwise  be  better  moisture  relations.  It  accordingly 
happens  that  in  another  habitat,  where  the  water  relations  may  be 
somewhat  better,  the  same  succulent  may  be  found  growing  quite 
independently  of  larger  forms.  Where  the  conditions  last  named 
are  not  met,  the  two  types  of  plants  seem  to  be  distributed  with  little 
relation  to  each  other.  When,  however,  either  occurs  alone,  there  is  a 
certain  adjustment  by  which  the  plants  come  to  be  fairly  equally 
separated,  and  thus  as  between  equal  areas  to  have  about  the  same 
population.  So  far  as  are  concerned  possible  reasons  for  the  correla¬ 
tion  of  size  and  numbers,  it  can  be  said  that,  other  features  being  equal, 
the  larger  the  size  of  the  individuals,  the  fewer  can  occupy  a  given 
area.  But  under  conditions  which  were  not  fully  worked  out,  it 
appeared  that  in  some  areas  where  there  is  fairly  intense  aridity  the 
shrubs  are  dwarfed,  as  might  be  expected,  but  at  the  same  time  they 
may  be  found  to  be  very  numerous.  So  anomalous  a  condition  was 
found  difficult  to  explain.  It  does  not  appear  improbable,  however, 
from  what  was  observed  of  the  behavior  of  such  species,  that  the 
limitation  in  the  amount  of  soil  moisture  worked  to  restrict  both  root 
and  shoot  development,  with  the  effect  that  in  such  areas  a  fairly 
large  population  of  dwarfed  non-succulents  is  supported. 

An  examination  into  the  local  occurrence  of  species,  with  the  neces¬ 
sarily  somewhat  close  study  of  plant  habits,  impresses  the  observer 
along  two  opposite  lines.  On  the  one  hand,  there  is  often  seen  to  be 
marked  diversity  among  perennials  as  to  habit  of  growth  even  in 
the  same  habitat,  but  on  the  other  hand,  species  belonging  to  different 
genera,  and  even  of  different  families,  may  be  strikingly  alike,  although 
superficially  so.  In  the  circumstance  last  named  the  veld  appears 
be  populated  with  but  a  single  species,  of  equal  height  and  of  the  same 


156 


FEATURES  OF  THE  VEGETATION  OF  THE 


general  appearance,  when  in  fact  monospecific  communities  do  not 
appear  to  be  common  in  the  arid  habitats  of  southern  Africa. 
Whether  rightly  so  or  not,  the  observer  is  likely  to  be  convinced  that 
at  least  where  the  growth  habit  tends  toward  the  uniform,  there  is 
manifest  in  the  fact  a  direct  adjustment  to  the  various  environmental 
factors  based  on  physiological  reactions.  In  the  last  analysis  this 
attitude  connotes  a  long  period  of  time  during  which  the  adjustments 
are  constantly  taking  place,  and  it  may  include  gradual  if  slow  en¬ 
vironmental  changes  of  the  habitat  itself,  in  this  case  moving  away 
from  the  humid  and  terminating  in  the  desertic,  in  which  extreme  the 
uniformity  of  habit  characteristic  of  the  mesoxerophytic  habitat  may 
be  lost. 

The  possible  leveling  of  plant  habits  in  the  intermediate  stages 
between  desert  and  mesophyllous  conditions,  such  as  are  observed  in 
the  Central  Karroo,  for  example,  offer  many  features  of  interest.  Thus 
among  the  marked  characteristics  of  the  flora,  as  above  remarked,  are 
often  superficial  resemblances  of  species  of  different  genera,  or  even 
of  different  families.  A  few  examples  may  be  cited.  At  the  outset 
there  may  be  pointed  out  instances  where  certain  species  of  the  Karroo 
veld  are  similar  to  species  of  arid  regions  elsewhere.  Thus  Euphorbia 
stellcespina  (plate  8)  of  the  Central  Karroo  has  a  close  resemblance  to 
small  species  of  Cereus  of  portions  of  southwestern  United  States. 
An  inspection  of  the  plate  will  reveal  the  presence  of  longitudinal 
furrows,  as  in  Cereus,  which  can  hardly  fail  to  have  a  similar  function, 
namely,  that  of  accommodating  the  plant  to  a  varying  store  of  water, 
through  which  the  tissues  expand  when  turgid  and  contract  on  suffi¬ 
cient  water-loss,  without  injury,  and  the  general  similarity  between 
many  other  species  of  Euphorbia  and  of  different  species  of  the  cacti  is 
common  knowledge.  Also,  the  Aloes  strongly  recall  species  of  Agave 
in  the  fact  of  a  common  leaf  succulence.  And  the  arborescent  Aloe 
dichotoma  on  the  plains  of  the  Southwest  Africa  has  many  points  of 
resemblance  with  Yucca  arbor escens  of  the  Mojave.  Not  to  extend 
the  list,  it  can  be  mentioned  that  species  of  Sarcocaulon  may  be  very 
like  young  Fouquieria  splendens.  The  resemblance  last  referred  to 
extends  in  certain  species  to  the  deposition  of  wax  or  other  material 
in  the  cortex.  So  far  as  conditions  on  the  veld  are  concerned,  it  will  be 
understood  that  there  may  be  both  marked  uniformity  as  between 
species  of  a  single  genus  and  marked  diversity  as  well.  While  nearly 
any  genus  may  be  cited  to  support  this,  it  is  preeminently  the  case  in 
Mesembryanthemum,  as  is  well  known.  Referring  to  the  frequent 
tendency  of  species  of  different  genera  or  families  to  have  a  similar 
vegetative  expression,  one  can  mention  species  of  Crassula  and  of 
Cotyledon,  which  are  sometimes  so  nearly  alike  that  only  close  inspec¬ 
tion  will  reveal  the  identity  of  each.  Also,  the  resemblances  between 
species  of  Senecio  (plate  10)  and  Euphorbia  (plate  13),  although  not 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


157 


close,  are,  however,  quite  evident.  Mention  can  also  be  made  of 
Stapelia  (plate  28d),  which  recalls  certain  species  of  Euphorbia 
and  certain  cacti  as  well;  and,  too,  there  is  also  a  likeness  between 
cushion  forms  of  Cotyledon  and  of  Mesembryanthemuin  (plate  11).  By 
including  the  non-succulent  types,  the  list  of  resemblances  in  plant 
types  might  be  much  extended.  Something  of  the  uniform  character 
of  much  of  the  flora  of  the  veld  is  shown  in  plates  6a,  6b,  12. 

With  such  leveling  processes  active,  one  can  well  understand  how 
in  course  of  time  there  may  result  a  certain  degree  of  floral  monotony 
as  to  perennial  woody  species,  such,  for  example,  as  is  characteristic  of 
much  of  the  landscape  of  South  Australia.  It  will  be  seen  that  such 
result  may  be  other  than,  and  quite  distinct  from,  the  occurrence  of 
one  species  only  in  a  single  habitat,  the  aspect  of  which  would  be  the 
same. 

The  physiological  reactions  associated  with  such  vegetative  modeling 
as  has  been  referred  to  are  various  and  complex,  affecting  both  the  root 
and  the  shoot.  It  has  already  been  shown  that  possible  differences  in 
capacity  for  excretion  of  watery  vapor,  as  between  unlike  species,  is 
of  importance  in  the  sorting-out  process  by  which  each  comes  to  live 
in  the  habitat  having  an  appropriate  water-supply.  A  low  capacity 
may  signify  possible  survival  in  an  arid  habitat.  Differences  in  the 
total  requirement  of  water  also  operate  toward  the  same  end,  that  is, 
toward  the  adjustment  of  species  to  the  habitat  and  to  each  other.  A 
great  moisture  deficit  of  the  air  occasions  a  rapid  rate  of  evaporation, 
so  that  water  taken  in  by  the  roots  is  divided  during  growth  as  to  its 
ultimate  physiological  fate,  a  relatively  large  amount  leaving  the 
surface  of  the  shoot  in  transpiration  in  proportion  to  that  fixed  in  the 
metabolic  processes.  Such  partial  starvation  restricts  shoot  growth  in 
all  ways,  but  particularly  as  regards  the  distal  members,  or  foliar 
organs,  which  are  most  intimately  affected  by  the  sub-aerial  environ¬ 
ment.  Short  shoots  and  small  leaves  are  a  consequence  of  this  group 
of  reactions.  There  are  also  immediate  structural  effects  which  should 
be  noted.  Owing  to  the  fact  that  under  conditions  of  aridity  anhy¬ 
drides  are  recognized,  the  cell-walls  of  xerophytes  may  be  heavy. 
This  remark  applies  to  the  outer  epidermal  wall  in  all  instances,  except 
when  there  is  a  cover  of  trichomes,  and,  to  a  less  degree,  it  is  true, 
several  other  tissues  of  the  leaf.  With  the  increase  in  the  thickness  of 
cell-walls  there  is  a  corresponding  decrease  in  the  living  cell-contents, 
and  this  directly  operates  to  cut  down  the  water  requirements  of  the 
species. 

Such  remarks,  it  should  be  noted,  apply  to  non-succulents,  especially 
to  xerophyllous  sclerophytes,  of  which  the  foliar  organs  are  not  only 
small  but  relatively  thick  as  well.  As  to  succulents,  it  has  been  de¬ 
termined  that  under  more  moist  conditions  mucilages,  with  high 
water-absorption  capacity,  and  not  anhydrides,  tend  to  be  formed. 


158 


FEATURES  OF  THE  VEGETATION  OF  THE 


The  permanent  foliar  organs  of  plants  of  this  type  appear  usually,  or 
always  when  mature,  to  have  low  capacity  for  moisture  excretion,  and 
in  possible  association  with  this  circumstance  the  roots  are  usually 
meagerly  developed  and  lie  close  to  the  surface  of  the  ground. 

Finally,  a  word  should  be  said  in  regard  to  the  reaction  of  the  roots 
of  xerophytic  species  of  different  growth  habit.  In  certain  succulents 
it  has  been  found  that  a  relatively  high  soil  temperature  is  requisite  for 
an  adequate  rate  of  growth,  and  since  in  all  species  an  appropriate 
supply  of  moisture  in  the  soil  is  a  prerequisite  for  growth,  it  accord¬ 
ingly  may  happen  that  succulents  occur  most  extensively,  or  only, 
in  regions  with  rains  in  summer.  But  so  far  as  the  root-systems  of 
non-succulents  are  concerned,  the  matter  is  otherwise.  There  are  in 
such  forms,  as  in  succulents,  specific  adjustments  to  the  temperature 
of  the  soil,  which  may  in  these  forms  include  low  temperatures,  of 
which  one  result  is  the  characteristic  placing  of  the  roots.  The  dif¬ 
ferences  in  the  reaction  of  roots  to  the  temperature  of  the  soil,  as,  for 
example,  in  slopes  with  unlike  aspect,  may  be  an  important  factor  in 
determining  the  local  distribution  of  the  species.  The  possible  dif¬ 
ferences  in  the  reactions  of  roots  of  diverse  species  to  the  oxygen- 
supply  of  the  soil  is  another  condition  of  much  importance;  and  this 
has  relation  to  the  temperature  at  which  the  root  best  grows,  and  may 
be  specific. 

Not  to  further  extend  the  list  of  reactions  of  a  species  of  the  arid 
habitats,  it  will  be  appreciated  that  they  are  immediately  concerned 
not  only  with  the  major  environmental  factors,  of  which  but  a  few 
have  been  alluded  to,  but  with  minor  factors  as  well,  and  the  important 
point  is  that  the  unlike  species  react  to  such  common  impinging  en¬ 
vironmental  factors.  Where  reactions  take  fairly  parallel  courses,  the 
results  are  to  a  certain  degree  harmonious,  and  to  the  degree  that  they 
are  so  the  species  tend  to  become  more  and  more  alike.  Thus,  as 
earlier  remarked,  it  is  not  difficult  to  appreciate  how  in  an  arid  region 
there  may  be  similarity  between  the  vegetative  parts  of  different 
species  and  how  by  the  association  of  those  with  most  harmonious 
reactions  there  may  be  formed  a  flora  with  superficially  similar  indi¬ 
viduals,  and  which  may  thus  be  fairly  monotonous,  or  how,  where  there 
are  fundamental  differences  between  species  as  regarding  metabolic 
processes,  as  between  succulents  and  non-succulents  to  cite  extreme 
types,  there  may  be  floral  diversity.  Such  are  the  logical  and  inevitable 
results  of  physiological  reactions  of  the  living  forms  to  so  pronounced 
an  environment  as  that  of  the  arid  portions  of  southern  Africa. 

A  word  should  be  said  in  conclusion  regarding  the  combination 
of  experimental  and  of  observational  work  in  general  studies  on  plants. 
It  needs  no  argument  to  support  the  statement  that  only  so  far  as  the 
reactions  of  plants  to  such  factors  as  temperature,  moisture,  oxygen, 
and  the  chemical  and  physical  conditions  of  the  soil,  are  studied  can  the 


MORE  ARID  PORTIONS  OF  SOUTHERN  AFRICA. 


159 


possible  adjustment  to  an  environment  possessing  these  factors  be 
reasonably  well  known ;  and,  in  order  to  apply  the  results  of  such  studies 
as  much  as  possible  should  be  known  in  regard  to  the  environment 
itself,  which  again  is  self-evident.  Such  physical  and  physiological 
investigations  are  particularly  desirable  in  association  with  an  arid 
habitat,  where  the  relations  are  largely  between  the  plants  and  the 
habitat,  rather  than  importantly  between  the  plants  themselves. 


THE  UBWH1  !’f 


OCi  G  1924 


UNIVERSITY  Of  ILLINOIS 


CANNON 


PLATE  1 


■ 


A.  W elwitschia  mirabilis  showing  character  of  habitat,  about  40  km.  east  of  Swakopmund, 

looking  toward  the  Swakop  River. 

B.  Acanthosicyos  horrida  in  bottoms  of  the  Swakop  River,  15  km.  east  of  Swakopmund. 


isnBsrrrw  mw®  imm 


CANNON 


PLATE  2 


A.  Welwitschia  mirabilis,  Namib  Desert,  about  40  km.  east  of  Swakopmund.  Female  plant 

B.  Welwitschia  mirabilis,  habitat  of  A.  Male  plant. 

C.  Zygophyllum  stapfii,  leaves,  natural  size. 


CANNON 


PLATE  3 


A.  Vegetation  in  Welwitschia  habitat,  about  50  km.  east  of  Swakopmund,  Namib  Desert.  Asclepias 

filiformis  (left),  Zygophyllum  stapfii,  and  Bauhima  marlothii.  Looking  south. 

B.  Vegetation  in  Welwitschia  habitat.  Looking  north,  toward  the  Swakop  River.  The  dark  shrub- 

lets  are  Zygophyllum  stapfii.  Asclepias  filiformis  (?)  is  in  the  bottom  of  the  shallow  wash. 

C.  Zygophyllum  stapfii  in  habitat  of  Welwitschia  mirabilis ,  about  40  km.  east  of  Swakopmund,  Pro¬ 

tectorate  of  Southwest  Africa. 


\ 


4 


CANNON 


PLATE  4 


A.  Arthrcerua  leubnitzii,  about  18  km.  east  of  Swakopmund,  Namib  Desert.  Plain  south 

of  the  Swakop  River. 

B.  Branch,  natural  size,  of  Arthrcerua  leubnitzii. 


iwwusrrr  of  Illinois  library 


CANNON 


PLATE  5 


A.  Tree  type  in  Low  veld,  near  Messina,  northern  Transvaal,  rainfall  about  25  inches,  of  which  about 

90  per  cent  is  in  summer.  Sesamnothamnus  lugardii  (?). 

B.  Adansonia  digitata,  near  Messina,  northern  Transvaal,  July,  showing  enormous  development  of 

stem  which  constitutes  a  water-storage  organ  of  great  capacity.  Rainfall  about  25  inches, 
of  which  about  90  per  cent  is  in  summer. 

C.  Euphorbia  cooperi  by  the  Zoutpansbergen,  rainfall  35  inches,  or  over,  of  which  about  90  per  cent 

occurs  in  summer. 


ajffiBSfflnrflF  ujMK  mtm 


CANNON 


PLATE  6 


A.  View  of  veld,  looking  southwest  from  kopje  near  Beaufort  West,  central  Karroo. 

B.  South  face  of  doloritic  kopje,  near  Beaufort  West,  Central  Karroo. 

C.  Bulboid  squat  stem  of  Adenia  schlechteri,  resting  freely  on  surface  of  ground,  with  water-storage 

capacity.  Low  veld  near  Messina,  northern  Transvaal.  Rainfall  about  25  inches,  90  per 
cent  in  summer. 


* 


flIIBWVKiMBUMm 


CANNON 


PLATE  7 


A.  Gymnosporia  buxifolia  (?)  on  kopje  near  Beaufort  West.  Used  in  transpiration  studies. 

B.  Branch  with  leaves,  half  natural  size.  Grewia  cana,  from  kopje  near  Beaufort  West.  Used 

in  transpiration  studies. 

C.  Leaves  and  spines  of  Gymnosporia  buxifolia  (?),  one-half  natural  size.  From  kopje 

near  Beaufort  West,  Central  Karroo. 


HfllBBfFF  OF  U1M8RS  LI2RAIU 


CANNON 


PLATE  8 


A.  Aloe  schlechteri  on  north  slope  of  kopje  near  Beaufort  West. 

B.  Euphorbia  stellaespina  on  north  slope  of  kopje,  near  Beaufort  West. 

C.  Quadrat  No.  1,  on  south  slope  of  kopje  (compare  plate  6b),  near  Beaufort  West.  The  following 

are  included  among  the  perennials  occurring  in  the  area:  Carissa  ferox  (?),  Euphorbia  mauri- 
tanica,  Grewia  cana,  Lycium  sp.,  and  Mesembryanthemum  sp. 


*»«¥fl®F¥  OF  MJLMttS  1138  ARY 


CANNON 


PLATE  9 


A.  Gasteria  disticha  growing  at  base  of  Euphorbia  mauritanica  (?)  on  upper  south  slope  of  kopje  near 

Beaufort  West.  Used  in  transpiration  studies. 

B.  Massonia  latifolia  at  base  of  Lycium  sp.  on  slope  of  kopje  near  Beaufort  West.  Used  in  transpiration 

studies. 


^ffFRsirr  of  HS8®  tmw 


CANNON 


PLATE  10 


A.  Crassula  quadrangularis  growing  at  base  of  Lycium  sp.  on  upper  face  of  kopje  near  Beaufort  West 

B.  Senecio  longifolius,  by  water-hole,  6  miles  east  of  Beaufort  West. 


^WTRinr  of  liieis  ubm» 


CANNON 


PLATE  11 


A.  Cotyledon  decussata  growing  at  base  of  Lycium  sp.,  Prince  Albert  Road,  Central  Karroo. 

B.  Mesembryanthemum  calamiforme-Cotyledon  hemisphcerica  (?)  community,  Prince  Albert  Road,  Central 

Karroo.  Rainfall  4.57  inches;  60  per  cent  in  summer. 


MTCRS1TY  8F  MJ.1S0MS  UBBAHY 


CANNON 


PLATE  12 


A.  Quadrat  No.  3,  near  Matjesfontein,  looking  toward  the  Wittebergen.  There  were  330  individuals, 

perennials,  on  the  area,  10  by  10  meters,  about  equally  divided  between  succulents  and  scle- 
rophylls. 

B.  Veld  near  Matjesfontein,  looking  toward  Ngaap  kopje.  Mesembryanthemum  spinosum,  M.  spp., 

Pentzia  virgata  dominate. 


niWBsmr  of  wm  imm. 


CANNON 


PLATE  13 


A.  Euphorbia  mauritanica,  veld  near  Matjesfontein. 

B.  Euphorbia  eustacei,  among  rocks,  near  Matjesfontein 


JIfITEBSUY  OF  UiHOtS  IHMtt 


CANNON 


PLATE  14 


A.  Acacia  karroo ,  in  winter  (August),  near  Matjesfontein. 

B.  Detail  of  A  showing  marked  spiniferous  character  of  branches,  which  is  especially 

noticeable  during  the  leafless  condition. 


iiirasnrfiF  ukb  mm 


CANNON 


PLATE  15 


A.  Mesembryanthemum  junceum,  on  veld  near  Matjesfontein. 

B.  Euryops  lateriflorus  by  rocky  outcrop  near  streamway,  Matjesfontein.  Used  in  transpiration  studies 


wtvTOsrrr  of  iluik»s  imm 


CANNON 


PLATE  16 


A.  Cotyledon  wallichii  on  low  kopje  near  Matjesfontein. 

B.  The  leaf  succulent  Cotyledon  coruscans  on  slope  of  low  kopje  near  Matjesfontein.  In  flower. 

One  specimen  of  C.  paniculata,  stem  succulent,  is  shown  in  middle  ground  at  left. 


CANNON 


PLATE  17 


A.  Crassula  perfossa  with  Cotyledon  orbxculata  at  back;  Grewia  cana  at  left.  Kopje  near  Matjesfontein. 

B.  Vegetation  on  north  slope  of  kopje,  near  Matjesfontein.  Euphorbia  mauritanica  in  middle  fore¬ 

ground,  with  Cotyledon  orbiculata  at  left  in  middle  ground;  Aloe  striata  (?)  in  middle  ground 
and  background.  Mesembryanthemum  spinosum  and  Crassula  perfossa  dominating.  Grewia 
cana  and  Lycium  sp.  at  left  and  right  in  background. 


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CANNON 


PLATE  18 


A.  Cotyledon  wallichii  on  veld  near  Matjesfontein. 

B.  Cotyledon  paniculata,  stem  succulent,  on  kopje  near  Matjesfontein,  M esembryanthemum 

junceum  (?)  dominating. 


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CANNON 


PLATE  19 


A.  Protea  neriifolia  at  Tweedside,  west  of  Matjesfontein,  used  in  transpiration  studies. 

B.  Vegetation  of  kopje,  near  Matjesfontein.  Mesembryanthemum  junceum  in  flower  in  middleground; 

Aloe  striata  (?)  with  flowering  stalks  on  either  side.  Small  specimens  of  Cotyledon  paniculata 
at  right  in  foreground  and  at  left  in  middle  ground.  Euphorbia  mauritanica  in  right  middle 
ground  with  Euclea  undulata  behind. 


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CANNON 


PLATE  20 


A.  Streamway  vegetation,  near  Matjesfontein.  Rhus  viminalis,  in  middle  ground,  with  Acacia  karroo, 

in  front,  as  a  shrub,  and  on  the  left  as  trees. 

B.  Lebeckia  psiloloba  on  edge  of  village,  Matjesfontein.  It  occurs  in  some  numbers  on  kopjes  a  few 

miles  west. 


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CANNON 


PLATE  21 


A.  General  view  of  quadrat  No.  4,  near  Matjesfontein.  Mesembryanthemum  spinosum  and  Pentzia 

virgata  dominant.  Asparagus  capensis.  Out  of  397  individuals,  perennials,  184  are  succulents. 

B.  Vegetation  on  lower  slope  of  foothills  near  Whitehill,  3  miles  east  of  Matjesfontein,  Central  Karroo. 

Euphorbia  mauritanica,  left  foreground;  Crassula  portulacea,  middle  ground;  Asparagus  sp., 
Rhus  sp.,  and  Euclea  undulata. 


jsratsirr  v  um»  ta#wi> 


CANNON 


PLATE  22 


A.  Root  exposure  of  Galenia  africana  (left)  and  Eriocephalos,  showing  characteristic  deep  penetration 

in  both  species,  with  prominent  development  of  superficial  roots  in  the  latter.  Matjesfontein, 
near  streamway. 

B.  Euphorbia  stolonifera  on  rocky  portion  of  veld,  near  Matjesfontein. 


iiJinnERsrrr  of  mwi-N  umm 


CANNON 


PLATE  23 


A.  Prominent  development  of  superficial  roots  in  Asparagus  sp.  on  veld  near  Matjesfontein.  Mesem- 

bryanthemum  spinosum  at  left  and  immediately  back  of  the  small  Asparagus  shoot. 

B.  Root  exposure  of  Euryops  lateriflorus  by  small  wash,  7  miles  west  of  Matjesfontein.  The  small 

shrubs  in  the  background  are  in  part  Galenia  africana. 


ajKWHBnr  of  iiiwots  mm 


CANNON 


PLATE  24 


A.  Superficial  and  fairly  meager  root  system  of  Euphorbia  stolonifera  exposed  in  part  by 

erosion,  with  Cotyledon  (?)  in  shadow,  and  Mesembryanthemum  spinosum  in  back¬ 
ground. 

B.  Exposure  of  roots  of  Cotyledon  canescans  by  small  wash  near  Matjesfontein.  There 

were  two  main  roots,  both  superficial,  with  numerous  short  roots.  One  of  the 
main  roots  lies  on  the  surface  of  the  ground  in  the  foreground,  and  the  other  is 
to  be  seen  in  front  of  a  sheet  of  paper  back  of  the  plant. 

C.  Root  system,  removed  soil,  of  Cotyledon  coruscans,  showing  its  meager  development. 

The  roots  are  mainly  superficial.  Veld  near  Matjesfontein. 


isWERSlTY  of  tmm  irnm 


CANNON 


PLATE  25 


A.  Euphorbia  multiceps  showing  prominently  developed  tap  root;  veld  at  Matjesfontein. 

B.  Euphorbia  multiceps-,  veld  at  Matjesfontein. 

C.  Aloe  variegata  in  flower;  veld  near  Matjesfontein. 


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CANNON 


PLATE  26 


A.  Root  system  of  Mesembryanthemum  junceum  showing  prominently  developed  superficial  roots. 

About  one-third  natural  size.  Veld,  near  Matjesfontein. 

B.  Mesembryanthemum  spinosum  showing  characteristically  marked  development  of  superficial  roots. 

Flats  south  of  Whitehill,  3  miles  east  of  Matjesfontein. 


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CANNON 


PLATE  27 


A.  Elytropappus  rhinocerotis,  near  streamway,  Matjesfontein,  showing  prominently  developed  super¬ 

ficial  roots,  of  the  generalized  root  system,  exposed  by  erosion. 

B.  Root  exposure  in  Lycium  sp.  growing  by  stream  near  Matjesfontein,  showing  vegetative  reproduc¬ 

tion  from  superficial  lateral,  and  strongly  developed  tap-root. 


^fflERSmr  OF  BJJWS 


CANNON 


PLATE  28 


A.  Anacampseros  papyracea,  one-half  natural  size,  Whitehall,  3  miles  east  of  Matjesfontein. 

B.  Androcymbium  sp.,  one-half  natural  size,  veld  at  Matjesfontein. 

C.  Crassula  columnaris  in  right  middle  ground,  in  flower,  and  in  preflowering  stage. 

Mesembryanthemum  pygmaeum  (?)  at  left.  About  one-fourth  natural  size,  veld 
at  Matjesfontein. 

D.  Stapelia  pillansii  on  low  outcrop  near  Matjesfontein. 


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CANNON 


PLATE  29 


A.  Haworthia  sp.  showing  the  fleshy  and  short  superficial  roots;  veld,  Matjesfontein,  natural  size. 

B.  M esembryanthemum  pygmaeum,  left;  young  Crassula  columnaris,  below;  Cotyledon  (?),  right. 

Two-fifths  natural  size. 


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CANNON 


PLATE  30 


A.  Young  plants  of  Cotyledon  paniculata,  natural  size,  showing  early  development  of 

succulency  in  the  stem;  veld,  near  Matjesfontein. 

B.  Crassida  lycopodioides;  veld,  near  Matjesfontein.  Natural  size. 


nWBsrrr  bf  um  uw«-. 


CANNON 


PLATE  31 


A.  T  oung  plant,  one-half  natural  size,  of  Cotyledon  wallichii,  showing  the  early  development  of 

succulency  in  the  stem  and  superficial  nature  of  meager  root  system;  veld,  near  Matjes- 
fontein. 

B.  Pelargonium  crithmifolium  showing  prominent  development  of  tap  root  one-half  natural  size; 

veld,  near  Matjesfontein. 


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